What Could Bleeding Post Sex Indicate?

Lately after sex, I've been bleeding a little. I don't know if it's because I'm kind of dry when we start or what could be the cause. It's never a lot of blood. It's actually a very small amount. Both I and my partner have been tested clean. Usually when we start, I'm kind of dry and it hurts until we get going. I was just wondering if it's something I should really be worried about.