I have some bad news for you. Most of that is not blood. It's the lining of the uterus and fragments other material. The body preps to have a baby, and if the egg goes unfertilized, then the body flushes all the junk out. Hence the cramps; they help the uterus to release the lining, etc.
Yeah, pretty nasty shit. Sex while on the period, okay. Eating a girl out, preeeeeeeetty icky.