How do we know that the blood of a woman is Tamei only if it emerges from its source in the womb)?
Rashi: We learn it from "v'Hi Gilsah Es Mekor Damehah" (20:18),
Why does the Pasuk add the phrase "Dam Yihyeh Zovah..."?
Rashi and Rashbam: To teach us that it is only red (blood) that is Metamei, but not white like the Zov of a Zav. 1
See Targum Yonasan, who lists other colors that are similar to blood-red and that are also Metamei. See also Ba'al ha'Turim.
Why is she called 'Nidah'?
How many sightings must she have in order to be Tamei for seven days?
Rashi: Even if she sees only once she is Tamei for seven days.
Why does a Nidah not bring a Korban?
Ramban (11): Nidah is normal. She was not healed from an illness.
The Torah did not say how a Nidah becomes Tehorah!
Da'as Zekenim, from Avodah Zarah 75b): "B'Mei Nidah" (Bamidbar 31:23) hints that a Nidah immerses in a Mikveh [after the seven days].