2,115. Attributing a Stain to External Causes
Hilchos Issurei Biah 9:23
If a stain is found on both the woman’s clothing and on her skin, then she may attribute it to all possible external causes, e.g., maybe she sat on a louse, which bled. This is the case up to a stain the size of a gris (about the size of a dime). If the stain is larger than this, she may no longer attribute it to a louse. Even if there’s a crushed louse on the stain, once the stain is larger than a gris it may not be attributed to a louse.
Hilchos Issurei Biah 9:24
A woman may attribute a stain to her son or her husband if he was occupied with blood, if his hands were dirty, or if he had a wound. She may assume that he touched her even though she didn’t know it and the blood came from him.