We have learned that with regard to the laws of ketamim we try to be lenient and are willing to attribute them to some outside cause rather than declare the woman to be a nidda (see daf 58). The Mishna on today’s daf teaches that we cannot always do so.
According to the Mishna, if a woman lends her cloak to a non-Jewish woman or to a nidda and then finds a bloodstain on it, she is not rendered a nidda, as the stain can be attributed to the woman to whom she lent her cloak. If, however, three women shared a cloak and then a bloodstain was found on it, all three are rendered ritually impure. The difference between these cases is that in the first case we can rule leniently for the owner of the cloak without affecting the status of the other women, for they are, in any case, ritually impure. In the second case, however, we know for certain that one of these women must be rendered a nidda, at least on a rabbinic level. Were we to rule leniently on behalf of one or two of the women, it would determine that we must rule stringently regarding the third woman.
The Mishna is not sufficiently clear regarding the first case, where the woman lends her cloak to non-Jewish woman or to a nidda. Rashi and the Rambam explain that the owner lends the cloak to the second woman and when it is returned she discovers the bloodstain after she has worn it herself. The Rashba argues that the owner first wore the cloak and then lent it to the second woman, discovering the bloodstain only when it is returned. According to the Meiri the owner first wore the cloak, lent it to the second woman, and then wore it again before finding the bloodstain.
The Ḥatam Sofer explains that Rashi may not disagree with the Rashba; he simply offers a case that is more novel than the others. He teaches that even in a situation where the bloodstain on the cloak is discovered in the owner’s possession after she has worn it, it can still be attributed to the woman to whom it was lent.