Gemara

(Mishnah): If Leah lent a garment to a Nidah or a Nochris, and found a Kesem (also Leah wore it), she is Toleh (attributes) that the Kesem came from the borrower.

If three women wore the same garment or sat on the same bench, and blood was found, all are Teme'os.

(Rav): She may be Toleh on a Nochris only if the Nochris saw blood.

Presumably, the case of a Nochris is like a Nidah (with which it is taught), i.e. she saw.

Objection (Rav Sheshes - Beraisa #1): She may be Toleh on a Nochris;

R. Meir says, she may be Toleh (only) on a Nochris who reached the age to see blood.

Even R. Meir requires only that she is old enough, but not that she actually saw!

Answer (Rava): You understand that R. Meir is more stringent than Chachamim. He is more lenient! (Rav holds like Chachamim.)

Contradiction (Beraisa #2): She may not be Toleh on a Nochris;

R. Meir says, she may be Toleh.

Resolution (Rava): In Beraisa #1, (Chachamim say that) she may be Toleh on a Nochris who has seen;

R. Meir says, she may be Toleh on a Nochris old enough to see, even if she did not see.

(Beraisa - R. Shimon ben Gamliel): She may be Toleh on a Shomeres Yom k'Neged Yom on the day after she saw, or on a Zavah counting seven clean days who did not immerse yet;

Rebbi says, she may not be Toleh. Therefore, both women are in Safek.

All agree that she may be Toleh on a Shomeres Yom k'Neged Yom on the day she saw, or on a woman during Yemei Tohar, or on a Besulah (who just got married. Her blood is Tahor).

Rishonim

Rambam (Hilchos Isurei Bi'ah 9:29): If Leah lent her garment to a Nidah, whether the Nidah was a Nochri or Yisraelis, and after getting it back Leah wore it and found a Kesem, Leah attributes it to the Nidah who wore it.

Magid Mishneh: Rashi explains that the Nochris is an adult, and she previously saw blood. It is possible that the Rambam agrees. Any Nochris who ever saw is forever a Nidah mid'Rabanan, for she never becomes Tehorah. The Ramban and Rashba explain like this

Kesef Mishneh: It seems that Rashi agrees with the Rashba, but the Rambam disagrees. The Magid Mishneh says that the Rambam could agree. This is wrong. If the Rambam held that she is Toleh even on a Nochris before the age to see, who saw, this is unlike the Rashba, and Rashi connotes unlike this. Since we must say that the Rambam disagrees with Rashi and the Rashba, why explain the Rambam unlike the simple meaning?! Also, the Magid Mishneh's reasoning is totally wrong. Based on his reasoning, if we know that a Yisraelis never immersed from Nidah, even if now she is Mesulekes Damim (not expected to see Dam Nidah, e.g. she is pregnant or nursing), she is Toleh on her! This is unreasonable, since she has not seen for a long time. Even though the Rashba holds that she is Toleh on a Nochris in such a case, this is because a Nochris is not normally particular about her blood and Kesamim. Therefore, if she once saw after maturing, even though she did not see while she had the garment, we are concerned lest her body and garments became dirty from her first blood, and they stained Leah's garment. We cannot say so about a Yisraelis, for Benos Yisrael normally clean their bodies and garments from blood, even if they do not intend to immerse. Rather, I explain the Rambam simply. She is Toleh on a Nochris only if we know that she was a Nidah while she had the garment. If she did not see then, even if she saw beforehand, she is not Toleh on her. This is what Rav taught, that the Nochris saw during the loan. He did not call her a Nidah, for she does not become Temei'ah.

Hagahos Maimoniyos (30): Just like she is Toleh on another woman, she is Toleh on herself. If she wore a garment when she was Nidah and did not check it, and wore it again when she is Tehorah or counting seven clean days, she is Toleh on when she was Nidah.

Ohr Some'ach: Also Sefer ha'Eshkol says so. The Rema brings from Sefer ha'Terumah that she is not Toleh in the first three of the seven days. This is a stringency that Ashkenazim accepted. If on the fourth clean day she wore a garment or sat on a sheet that she wore on sat in while she was Nidah, and found a Kesem, Acharonim argue about whether we are concerned that it was from the first three of the seven days. (I.e. she wore it also during the first three days, just she did not find the Kesem until day four - PF.)

Poskim

Shulchan Aruch (YD 190:41): If Leah wore a checked garment, took it off, and lent it to a Yisraelis Nidah, or a Nochris who reached the age to see blood and saw once, even if the borrower did not see blood while the garment was with her, and afterwards a Kesem was found, Leah attributes it to her.

Beis Yosef (DH v'Da): When we say that Leah attributes it to her, we do not distinguish whether Leah is totally Tehorah, or if she is counting seven clean days, for then she has Chezkas Tehorah. The Rashba says so.

Taz (31): The age to see is 12 years. The reason is because there is no loss if we attribute the Kesem to a Nidah or Nochris, but it would be a loss for a Tehorah Yisraelis. This is like YD 111:1. If we are unsure whether an Isur mid'Rabanan fell into Heter or Isur, we are Toleh that it fell into the Isur.

Shach (48): If she lent it to a Yisraelis Tehorah, both of them are Temei'os. Rashi explains like this, and it is obvious.

Atzei Levonah (31): The Mishneh l'Melech (9:29) and Chavas Da'as (Chidush 50) say that if the Nidah was pregnant or nursing, she is not Toleh on her, for she is Mesulekes Damim. Sidrei Taharah (70) said that even so she is Toleh on her, for the Nidah's source of blood is open.

Shulchan Aruch (ibid.): the same applies if Leah lent to her a checked garment, and after getting it back Leah wore it and found a Kesem, Leah attributes it to her, even if Leah is counting seven clean days.

Beis Yosef (DH Badkah): The Rashba says that in the Mishnah, Leah wore the garment, then lent it, and then found the Kesem. Rav does not discuss a Nochri who found a Kesem while the garment was with her, for if so, even if she were a Yisraelis, Leah would be Toleh on her, for this is like lending to a Nidah! Rather, the Nochris is an adult who reached the age to see and already saw Dam Nidah. He excludes a Nochri below the age who saw. He says that the Nochris is like a Nidah, but not exactly like her. We equate them as much as possible, i.e. the Nochris saw, just like the Nidah saw now.

Atzei Levonah (31): The Shach (50) explains that the Shulchan Aruch discusses when Leah took off the garment and checked it, and then lent it. Sidrei Taharah 72 (and Toras ha'Shelamim 39 - PF) asked why we must say that she wore it before she checked it. Since she checked it afterwards, the first wearing does not affect anything! I answer that we infer about one who wore a checked garment, removed it, and lent it to a Nidah without checking it and later wore it again. Then, we would not be lenient to be Toleh on the Nidah, since it is possible that Leah saw beforehand or afterwards.

Rema: See below 196:10, that during the first three of the seven days we are not Toleh on something else.

Shach (51): If the Rema holds like the Mechaber, he must say that during the first three days she is not Toleh even on herself. Normally, she may be Toleh herself (to say that the Kesem was from when she was Nidah - Sa'if 44). If so, the Rema should have commented also in Sa'if 44! Rather, it seems that the Rema holds like he wrote in 196:10, that during the first three days she is not Toleh on something else, but she is Toleh on a wound on her body that emits blood. If so, when she was Nidah, this is like a wound on her body that emits blood. However, I wrote (196:13) that she is not Toleh even on a wound on her body that emits blood. If so, in Sa'if 44 she is Toleh on when she was Nidah only after the first three of the seven days.

Shach (52): We are not Toleh only if it is more than a Gris. If it is less than a Gris, we are Toleh even in the first three days.