I was with a provider for the first time, it was a 1-hour incall, we were about 45 minutes into it and had done everything: showered together, 69 for what seemed like forever, put on the condom, doggy style, reverse cowgirl, cowgirl. We were finishing up on missionary, she kept getting wetter and wetter. The more slippery she got, the better it felt, it sounded real squishy and sloppy and kinky. We were both really into it, really going at it fast and hard. I reached down to play with her clit, everything between our bodies was soaking wet, I was amazed at her natural lube, I thought she responded greatly. She said to me, "Make sure the condom is still on," so I felt through all her moisture around the base of my dick, the condom had not slipped off.  I pulled my hand out and went to raise myself up to get a different angle of thrusting into her, so I put both my hands on the pillow that was under her head.  I saw a big bloody handprint on the pillow and asked stupidly, "Where did THAT come from?" Duh, idiot, it's your own damned hand! We stopped mid-stroke, I pulled my dick out of her pussy, and a trickle of blood followed it. The condom was stained a pink color too.  We jumped up in horror and found out that she had started her period. Her face turned redder than our nether regions. When I had been eating her pussy, not one sign of Aunt Flow.
We showered, the whole time she kept apologizing over and over, while I kept assuring her over and over that everything was going to be okay. The mood killed, it was the end of our date together. One and done.