What Is The Cause Of Post Coital Bleeding ?

I ve had sex with my partner multiple times before, but he had moved out of state last year, so we haven t been intimate for over a year. He s a pretty well endowed, over 6ft tall 11in long and thicker than your average, almost comparable to a Gatorade cap. I on the other hand am fairly small, under 5 ft and thin. We recently did the deed and we noticed slight bleeding afterward. This has never occurred before, and I haven t been with anyone since he left. While he was gone my pap smear tested negative, so what could be the cause of my postcoital bleeding?