1. A woman normally is forbidden to her husband on the day she expects her period, unless she examines herself and finds that she is Tahor.
2. A woman who is counting seven clean days is presumed to be Teme'ah during that time.
3. Rav Yehudah explains that the Mishnah is discussing three times of the month.
4. Rav Chisda understands that the Mishnah follows the view of Rebbi Meir.
5. The Gemara discusses a case of a woman who has a change in her Veses Kavu'a.
A BIT MORE
1. Rebbi Meir: The exception to this rule is a woman in hiding. Her menstrual blood is presumably diminished due to her fear, and therefore she is permitted (as long as she does not notice that she is menstruating) without even having to examine herself, despite the fact that the day arrived on which she usually experiences her period.
2. This is true even if she is counting clean days during the eleven days after the Nidah cycle.
3. Rav Yehudah: During the seven days of Nidah, a woman is clearly Teme'ah, and during the following eleven days (after immersion) she does not have to check herself and is presumed Tehorah. During the rest of the month she is presumed Tehorah, but l'Chatchilah she should examine herself to be sure that she is Tehorah.
4. Rav Chisda: The Mishnah is teaching Rebbi Meir's law that a woman who does not have a Veses Kavu'a is forbidden to be with her husband during the rest of the month (after the eleven days after immersion). In fact, Rebbi Meir maintains that a man must divorce such a woman, lest she end up thinking they may have relations during this time and she in fact becomes a Nidah (while they are having relations).
5. If a woman always sees her menstrual blood fifteen days after she immerses (for Nidah), and one time she sees it twenty days after she immerses, the next month she must suspect that she will menstruate fifteen and twenty days after her most recent immersion.