Random bleeding during sex

Last night my boyfriend and I were having sex and we found out I was bleeding, like the beginning of a period. The problem is, with my birth control, I don’t get periods, and I haven’t changed anything since I started on the pill over a year and a half ago. We were using a condom and hadn’t changed anything that we usually do. There were no signs of the start of a period before we saw it. Any ideas on what might have caused this?