It could be any number of things.
Was sex particularly rough? You might have a cut somewhere that's still healing.
Have you had a pap smear recently? Bleeding after sex could be due to cervical changes that need to be checked out. Getting a pap smear every year keeps you on top of that.
Did you just start new birth control? Spotting is somewhat normal in the beginning.
I suppose it's also somewhat maybe possible it's implantation bleeding if you possibly got pregnant, but I'm more doubtful on that one. Many women never have it and you're on the pill. Not that accidents don't happen, of course.
If it continues or you are concerned now, call your doctor. I'm sure they will happily see you.