Blood in my underwear, what can it be?

I just had sex, for the second time, about two days ago. The first time I had sex, my cherry had popped. But since I just had sex two days ago, Im having some blood in my underwear, and I don't know why. I just ended my period about a week ago so it cant be that. Im a very paraniod girl, and feel like somethings wrong. Do you know of anything that would be wrong , or why this is happening?