Many years ago my then GF had a headache and I'd read that orgasm would help reduce her pain. So being the considerate BF, I offered to help her out.
After giving her oral we went on to intercourse. She did then orgasm, and when I pulled out, I saw my crotch as well as hers was covered in blood. We both realized that she'd just then started her period. That poor girl was so embarrassed and apologetic. I laughed it off and told her not to worry, that it was a natural thing. I was just happy that her headache was gone... LOL!