2,104. Biblical and Rabbinic Laws
Hilchos Issurei Biah 9:1
Under Biblical law, a woman isn’t rendered ritually unclean as a niddah or a zavah until she feels a sensation, menstruates and sees blood that has emerged from within her flesh as has already been explained. She is only rendered impure as of the time she menstruated. If she doesn’t feel a sensation, but rather she conducts a self-inspection and finds blood in her vaginal canal, we assume that it was accompanied by a sensation, as has also been discussed.
Hilchos Issurei Biah 9:2
By rabbinic enactment, if a woman sees a blood stain on her skin or clothes, she is rendered unclean the same as if she found blood within her flesh. This is so even if she didn’t feel a sensation and even if she made a self-inspection and didn’t find blood. This is a stringency based on doubt because the stain may have come from uterine blood.