When women are menstruating, we are not supposed to do the inverted yoga poses, like headstands or shoulder stands or anything where blood flows, um, "upstream." My yoga buddy and I have come to euphemise "that time of the month" as the time when we are Not Inverting. As in, "I feel crampy, I guess I won't be inverting soon" or "Why are you being such a bitch?! Are you not inverting?"