Commentary for Niddah 8:2

ותולה (she hangs it/blames it) – the bloodstain that she found.

[A woman] may attribute [a bloodstain] to any [external] cause to which she can possibly attribute it.
If [for instance] she had slaughtered a beast, a wild animal or a bird,
Or if she was handling bloodstains or if she sat beside those who handled them.
Or if she killed a louse, she may attribute the bloodstain to it. How large a stain may be attributed to a louse? Rabbi Hanina ben Antigonus says: one up to the size of a split bean; And even if she did not kill it.
She may also attribute it to her son or to her husband.
If she herself had a wound that could open again and bleed she may attribute it to it.
Section one: If a woman sees a bloodstain on her body or on her cloak in a place where she must suspect that it could be menstrual blood but there is some external circumstance to which she could attribute it, she may attribute it to this source and remain pure. It is interesting that along with all of the stringencies the rabbis initiated when it comes to menstrual blood, they also instituted many leniencies, including those we see in this mishnah.
Sections 2-3: The mishnah now gives some examples of possible things she may have been doing that would allow her to attribute her bloodstain to another source.
Section four: Here we can see just to what extent the rabbis ruled leniently. Even if all a woman is doing is killing lice, which probably don't usually bleed that much, she may attribute a stain up to the size of a split bean to a louse. Rabbi Hanina ben Antigonus disagrees with the first opinion and holds that even if she is not the one killing them, she may attribute the bloodstain to the blood of the louse.
Section five: She can also attribute the stain to her husband or son for instance if they slaughtered an animal or did something else connected to blood.
Section six: Finally, she can also attribute the blood to her own wound, even if it was already scabbed over. As long as it is possible for the wound to open and bleed she can attribute her bloodstain to a wound. Note that this is so even if it doesn't look like the blood came from that wound.

בכל דבר שהיא יכולה לתלות – to state that it (i.e., the bloodstain) was not from here.

מאכולת (louse) – vermin.

עד כמה הוא תולה – with the blood of a louse/vermin.

עד כגריס (the size of a bean/split bean) – but more than this, the blood of a louse is not that great. And in this, however, she blames, and even though she didn’t know that she killed it. But Rabbi Hanina [ben Antigonos] disputes the first Tanna/teacher who stated, that yes (i.e., she may place the blame upon it), if she killed it, but not, if she didn’t. But the Halakha is not according to Rabbi Hanina ben Antigonos.

ותולה בבנה ובבעלה – that lie next to her, if they have a wound, to state that this bloodstain was from them.

ואם יש בה מכה – that the skin/membrane lifted off [the wound].

והיא יכולה להתגלע (and she is able to scratch it open) – to be revealed. From the language (Proverbs 17:14): “Before a dispute flares up/לפני התגלע הריב [drop it]," that is to say, that he should the skin/membrane and peel it and the wound will be revealed and blood comes out from it, she may blame it (i.e., rely upon it).