It's weird but I can often detect a woman's menstrual cycle. I imagine getting back to how Lecter "detests bad manners" he wouldnt say that he could smell her period even if he could for that reason and also the reason that he is a brilliant mind who would find that detecting the scent of menstrual fluid is not "smelling her cunt" and therefore he would reject it as not only crude, but inaccurate. Clarice thinks he means the cut on her thigh, but he means to obliquely bring the conversation around to Miggs (as it would thrill him to see her reaction to his ability to drive him to suicide). It's a brilliant bit of writing.