The practice of the daughters of Israel involved with ritual food was to cohabit with two clothes prepared for examination, one for him and one for her. The pious ones had a new cloth for every cohabitation, if there were multiple cohabitations.
If blood was found on his cloth, then she indisputably had blood during cohabitation, and each one is now obligated in a sin-offering. Regarding her cloth, there are three time periods. If blood was found on her cloth immediately after cohabitation, both need a sacrifice. If it was found after a time (enough to reach under a pillow and pull out a cloth), both bring an offering of uncertainty. If it was found after that, there is not more consequence than making the foods that she touched ritually impure retroactively.
But why is the above true? Maybe it was the blood of a louse on the cloth? Rabbi Zeira said, “That place is considered inspected with regard to a louse” – meaning, lice never visit the genital area. Others say, “It is too crowded for a louse,” - meaning, lice are usually not found in the genital area, because it is too constricted for them to enter. What is the practical difference between the two explanations? – Where a crushed louse was found on a cloth. According to the first explanation, it must have come from elsewhere, and the blood came from the woman. But according to the second, perhaps the louse was in the genital area, and then the organ crushed it, so the man and the woman are in doubt about their transgression.
Art: Amedeo Modigliani - Dark Young Woman Seated by a Bed