Maybe she's setting up the mikvah for the next time she has sex, now that she's used up the last one?
/ no clue about Jewish rituals.
Basically, the way it works is, no sex during your period or for a week after. Then you go to the mikvah, then you can have sex again, until your period starts again the next month. (All this is supposed to be within marriage.)
Reasoning for why varies, depending on who you ask. My favorite explanation was one I heard from the rabbi at Hillel when I was in college. There are a few different times that people go to the mikvah. It's usually thought of as a women's thing, since women go most often, since they have to go every month. Women also have to go after giving birth, or having a miscarriage. Anyone, male or female, has to go after attending to or touching a very sick person or a corpse, or after touching the Torah.
There are a few other times, too, but the things that all of them have in common, other than the Torah one, is that they're times when people come in very close contact with birth or death or conception -- lives entering the world and lives leaving the world. These are holy things, closer to the realm of G-d than the realm of people, and thus should be approached with caution. (Not scared caution, but reverential caution, the sort of "this is something special, let it be OK" caution.) The Torah, as the word of G-d, is also in that holy realm -- generally, Jews don't touch the scroll itself with our bare hands. We move it around by the wooden handles, and when reading it, we use a special silver pointer to keep our place, so that we don't touch the actual scroll with our fingers. So accidentally touching it is also getting much closer to that holy realm than people usually do.
Going to the mikvah, then, is a way of bringing ourselves back into the proper place in this world, reminding ourselves that, while there is a world to come and all kinds of holiness out there, we've got plenty that we need to do in the ordinary physical world first.
t edit: this is mostly stuff remembered from what I heard the rabbi say about seven years ago. I might have a few details wrong.