Gemara

(Shmuel): If a woman checked the ground, sat on it and found blood there, she is Tehorah. "Bi'Vsarah" teaches that she is Teme'ah only if she felt it in her body.

(Beraisa): If a woman checked himself after Bi'ah and found blood, they do not bring a Korban. Their Tum'ah is Safek.

Question: If she was Margish (during Bi'ah), why is the Tum'ah Safek if she checked afterwards?!

Question (against Shmuel - Mishnah): If a woman found a Kesem on her body facing the place, she is Teme'ah.

This is even if she was not Margish!

Answer #1 (R. Yirmeyah mi'Difti): Shmuel teaches that (without Hargashah) she is Tehorah mid'Oraisa. He agrees that she is Teme'ah mid'Rabanan.

Answer #2 (Rav Ashi): Shmuel holds like R. Nechemyah:

(Mishnah - R. Nechemyah): Kesamim apply only to things that are Mekabel Tum'ah.

Question: According to Rav Ashi we understand why Shmuel discusses a woman who found blood on the ground. However, according to R. Yirmeyah, he should have discussed (the usual case,) a woman who found blood on her garment!

Answer: Shmuel teaches that not only are we Metaher (mid'Oraisa) when it is found on her garment, which cannot be checked so well, and perhaps the blood came from elsewhere. Rather, even when it is found on the ground, which can be checked well, and surely it is from her, we are Metaher!

58b (Mishnah): She may be Toleh a Kesem on anything possible, e.g.: if she slaughtered an animal or bird, or engaged in Kesamim, or killed a louse...

If she has a wound that can open up and emit blood, she is Toleh on it.

A case occurred in which Leah found a Kesem. R. Akiva asked if she had a wound.

Leah: I had a wound, but it healed.

R. Akiva: Perhaps it could open up and emit blood!

She said yes. R. Akiva was Metaher her. His Talmidim looked bewildered.

R. Akiva: Chachamim enacted to be Metamei women due to Kesamim, but to be lenient (whenever we can be Toleh);

The Torah says "v'Ishah Ki Siheyeh Zavah Dam Yihyeh Zovah bi'Vesarah" -- (she is Tamei due to) blood, but not due to a Kesem.

59a - Contradiction (Beraisa): Chachamim enacted Kesamim, to be stringent.

Resolution (Ravina): The Beraisa means that Chachamim enacted Kesamim to be more stringent than the Torah. The entire law of Kesamim is mid'Rabanan.

61a (Mishnah): If three women slept in a bed and blood was found under the middle woman, if all of them checked and found themselves Tehoros, all are Teme'os.

Rishonim

Tosfos (58a DH Modeh): Rashi says that Shmuel admits that she is Temei'ah mid'Rabanan, for we assume that she had a Hargashah, but did not pay attention to it. This is wrong. Even if we are Muchzak that she had no Hargashah, she is Temei'ah, since she saw blood. Rav Ashi agrees that when she was not Margish, she is Temei'ah mid'Rabanan. Rav Ashi merely teaches that even if she found on the ground, she is Tehorah even mid'Rabanan.

Terumas ha'Deshen (246): If a woman felt her Makor open to emit blood, a checked afterwards and did not find anything, she is Temei'ah, for surely at least a drop the size of a mustard seed left, and it was wiped away or dissolved. Hargashah is a Torah Sevara (reasoning), when there is nothing to be Toleh on. This is clear from the Gemara's question 'if she was Margish, why are they exempt?!' This must be mid'Oraisa, since they bring a Korban. Since this is a Torah Sevara, just like according to the opinion that Vestos mid'Oraisa, even if she checked and found herself Tehorah, she is Temei'ah, the same applies to Hargashah.

Poskim

Shulchan Aruch (YD 190:1): Mid'Oraisa, a woman is not Temei'ah or forbidden to her husband until she feels that blood left her flesh. Chachamim decreed about a Kesem found on her body or garments, that she is Temei'ah and forbidden to her husband even if she did not feel.

Beis Yosef (DH Devar): Rashi explains that Chachamim decreed about Kesamim to be stringent about Divrei Torah, lest people be lenient about a real Nidah, but Kesamim themselves are mid'Rabanan, so we are lenient about them. We attribute them to anything possible.

Taz (1): Even if she sees blood, she is Tehorah (mid'Oraisa) if she did not feel it. The Magid Mishneh says so.

Question: A Kesem is a Sefek-Sefeka! Perhaps the blood is not from her. And even if it was from her, perhaps it was not from the Makor! We find such a Sefek-Sefeka in 187:5.

Answer (Taz 2): Most blood from a woman without a wound is from the Makor. Therefore, this is not considered a Safek at all. Tosfos (Kesuvos 9a DH v'Iy) says so. The case of the Rema (187:5) is different, for there she has a wound, and perhaps the blood is from the sides.

R. Akiva Eiger: Chachamim decreed about Kesamim even if she never saw blood until now, if she reached the age of Na'arus.

Shulchan Aruch (ibid.): This is even if she checked herself and found herself to be Tehorah.

Gra (3): We learn from the Mishnah (61a).

Shulchan Aruch (ibid.): If a woman felt her Makor open to emit blood, and checked afterwards and did not find anything, some say that she is Temei'ah.

R. Akiva Eiger: Minchas Yakov says that if she is pregnant, she is Tehorah, just like she is not concerned (for her Veses) even according to the opinion that Vestos mid'Oraisa. I say that one can distinguish. From what Tif'eres l'Moshe said about a pregnant woman who felt her Makor open to give birth, it is clear that he argues with Minchas Yakov. Also Noda bi'Yehudah (2 YD 120) says so.

Pischei Teshuvah (1): Sidrei Taharah brings from his Rebbi (Shev Yakov) that if a woman saw drops of blood on a white fluid, this is called without Hargashah, for we attribute the Hargashah to the Leichah. Also Beis Yakov brought this, but he disagreed.

Pischei Teshuvah (2): Panim Me'iros (1:127) says that Chachamim decreed Kesamim only on her garments, but not on a man's garment, even on his undergarment after he had Bi'ah with her, unless he used it to clean himself. Rav Yechiel Halir refuted this from the Tosefta (6:12), which says that if her son was lying next to her and a Kesem is found on his garment, she is Temei'ah. The same applies to all men; Chachamim discussed a typical case.

Pischei Teshuvah (3): Shev Yakov (45) brings a case in which a woman sneezed forcefully in the morning, and assumed that some urine exuded. She could not check at the time, and forgot about it until night, and then saw a small Kesem less than a Gris on her garment. Is she concerned for less than a Gris, since it came with Hargashah? He answered that she are not concerned, since it is less than a Gris. We are Metamei when she felt the Makor open and did not find anything. There is different, for she checked immediately and did not find anything. We must say that the Makor did not open for nothing. We must say that what came out was lost. Since Hargashah is mid'Oraisa, we are concerned lest a drop of blood came out. Here, she did not check until evening. Sneezing very often causes urine to exude from a woman. Had she checked immediately, she would have found wetness of urine. We do not establish an Isur (without a proper basis).

Pischei Teshuvah (3): His Talmid Tif'eres Tzvi (183:1) says that this is only in such a case, when drops exuded from her. Since she found only one small Kesem, this proves that it was urine. Had it been blood, much would have been found. However, if she felt that a little left, and does not know whether it was blood or urine, we attribute even less than a Gris to the Hargashah, and she is Temei'ah. Sidrei Taharah disagrees. Rather, even in such a case we are lenient. Terumas ha'Deshen was stringent only when the Makor opened, but not when she felt something exude from her. We attribute this to urine, which is common. Chachmas Adam (103:3) says that the same applies to a woman who often sees a white Leichah and is Muchzekes for this. Even if she felt something moist flowing and did not check, she is Tehorah, since she is Muchzekes for this. We attribute to what is common.

Pischei Teshuvah (4): Chachmas Adam (103:1) says that if her body shook and she checked and did not find anything, she is Tehorah. We attribute this Hargashah to other matters.

Pischei Teshuvah (5): Chavas Da'as says that even if she checked herself within Shi'ur Veses and did not find anything, she is Temei'ah. The Kreisi u'Pleisi says that this is only if she checked herself after Shi'ur Veses. It seems that Sidrei Taharah agrees.

Yad Avraham (183:1): Tif'eres Yisrael (on Kreisi u'Pleisi 183:1) says that if she knows that blood came from her body without Hargashah, she is Temei'ah due to a Kesem even for a tiny drop. Sa'if 6 supports him. (Some say that there is no Shi'ur for a Kesem on her body.) Minchas Yakov disagrees