1. Hargasha - הרגשה

(1) Regarding a woman who saw a [blood] stain on her body, if it was opposite her genital area she is impure; and if it was not opposite her genital area she is pure. If it was on her heel or on the tip of her big toe, she is impure. Regarding if it was on her thigh or on her feet, if on the inner side she is impure; and if on the outer side she is pure; and if on the sides, this side or that [i.e. the front or he back], she is pure. If she saw [a bloodstain] on her robe, if it was from the belt or below she is impure; from the belt or above, she is pure. If she saw it on the sleeve of the robe, if it reaches to opposite her genital area she is impure; and if it does not she is pure. If she would take it off and cover herself with it at night, if she finds a stain anywhere she is impure, because it rotates. And similarly regarding headscarves [which she covers her hair with at night, she is impure if she finds a stain anywhere on them, since they move around at night].

(2) A woman [who found a stain] may attribute it to any cause to which she is able to attribute it: [for instance] if she had slaughtered a beast, or a wild animal or a fowl, or if she was handling [blood] stains, or if she sat beside those who were handling them. If she killed a louse she may attribute [the bloodstain] to it. Up to how large a stain may be attributed [to a louse]? Rabbi Chanina ben Antignos says: up to the size of a split bean, and even if she did not kill it. And she may attribute it to her son or to her husband [i.e. if one of them has a wound]. If she has a wound that could open and produce blood she may attribute [the bloodstain] to it.

(3) It happened that a woman came before Rabbi Akiva. She said to him, "I saw a [blood] stain. He said to her, "Perhaps you had a wound?" She said to him, "Yes, but it has healed." He said to her, "Perhaps it [was a wound that] could open up and produce blood." She said to him, "Yes." And Rabbi Akiva declared her pure. He saw his students looking at one other [in astonishment]. He said to them, "Why is this matter difficult in your eyes? For the Sages did not state this matter to be stringent, but rather to be lenient, for it is said, 'And if a woman has a flow, blood will be her flow within her flesh,' (Leviticus 15:19); blood [renders a woman impure, according to the Torah], but not a [blood] stain."

(3) If a woman lent her robe to a gentile woman or to a niddah [a female who has menstrual discharges which render her impure], she may attribute [any bloodstain] to her. If three women wore the same robe or sat on the same bench, and blood was then found on it, they are all impure. If they sat on a stone bench or on [a ledge in] the colonnade of a bath house, Rabbi Nechemia considers them pure, for Rabbi Nechemia would say: Anything that cannot be rendered impure cannot retain a stain.

מדרבנן - perhaps she felt it and didn't know:

(1) A woman who has a discharge of blood from her uterus, whether due to outside factors or purposefully, is tameh (impure) if she felt it flowing out. If she felt the discharge she is tameh even if the blood did not flow out of her body, and even if she only saw a drop of blood the size of a mustard seed; she must observe seven clean days for it {Rema: as will be explained in Siman 196. And there is no distinction between a married and an unmarried woman with regards to the laws of niddah (Rivash in Siman 425 as brought by Bet Yosef), because all who have intercourse with a niddah are liable for karet.}

(1) According to Torah law, a woman is not rendered teme'ah and is not forbidden to her husband until she feels a sensation that blood left her body, but the sages decreed that upon seeing a blood stain on her body or clothing that she is teme'ah and forbidden to her husband even though she did not feel the blood leave her body, and even if she examines herself [afterwards] and finds herself clean. She requires a hefsek tahara to examine herself and find she is clean, and afterwards to count seven clean days from the day of her examination, as though she had certainly seen [niddah blood] as will be explained later in section 196. And if she felt that her uterus opened to release blood but checked herself afterward and did not find anything, there are those who say she is still teme'ah.