How much and for how long did it bleed. Was it not your period? Sex can make it start a bit sooner.
It is possible that sex can cause micro tears from stretching and friction, and thus light bleeding. Bumping your cervix can also cause bleeding.
Being fully aroused, lubricated and relaxed are very important. It might be difficult for you due to your history to trust your partner and get in that state.
It might be best for you to go for an ObGyn exam.