You said it yourself: "rough." Any activity that is rough enough to draw blood is dangerous. I suggest that you and your boyfriend negotiate what kinds of activities are safe and fun for you and which ones to avoid. Vaginal tissue is very delicate and easily injured; in addition, it provoids an optimum site for infections. So please play safe. Dr. J
I've experienced the same before. Actually, it happened not only once to me.
I agree with Janice M Epp, PhD.
It was rough and that's the only reason I can think.
Your organ during sex was not yet smooth and your partner keep on pumping so the tendency will be bleeding.