Guys and gals, I'm sorry to screw with anybody's fantasies here, but this is an old trick.
There's quite a couple ways ways a girl can fool a guy into thinking she's a virgin. Akiyuki Nozaki, in his novel Introduction to Anthropology (later translated as The Pornographers and turned into a movie) noted that a girl could discretely inject menstrual blood into herself before the act. Also, a girl will bleed a small amount on the day (or the day before) her period. Practicing kegel exercises helps, although no blood is produced. There's even surgical procedures (for girls marrying traditional guys who demand to marry a virgin).