If you've experienced blood loss after sex, you're in a state of complete anxiety... like one in four pregnant women.
Rest assured, most of the time it's benign. Your cervix will remain fragile throughout your pregnancy, so it may bleed if it's a little irritated.
It's rare, but it's possible that you'll have a little blood at about the time that your period would have been due. It's not quite the same thing if you bleed at the beginning or end of your pregnancy.
In any case, go see a doctor.