One, you could have had a small tear while
having sex. Or you could have implantation bleeding which is a sign of early pregnancy, it's rare, but it's a possibility. Id take a pregnancy test if I were you, if you're not pregnant, it's probably just some irritation from sex, which you don't need to worry about unless you're really in pain is see a doctor. But you should start wearing a condom, get on the pill, or at least pull out!