Hi!
Bleeding after intercourse is called post coital bleeding, and can have many causes. It could just be the tail end of your period. It could be a sign of an infection which can make your cervix senstitive. We always do a Pap test to rule out cervical cancer when someone comes in with this complaint too.
I usually don't recommend rushing in for a one time occurrence, unless you are due anyway, but certainly if this happens frequently, you should be checked!
Good luck!
Dr B