after sex is a fairly common thing. Even so you should still be seen by a doctor
to make sure that your bleeding does not represent something more serious. Bleeding from your vagina after sex can mean one of a few things. First, the blood could be coming from your uterus. This is common if you are having sex near the time of your period. Of all the causes, this is probably the most common. Another possibility is that the blood you are seeing is coming from your cervix. The cervix contains a lot of blood vessels that are easily broken especially during rough sex. The final possibility is that the blood is coming from the walls of your vagina. This can occur during sex that is rough without the proper lubrication. Blood from the vaginal wall is usually quite painful and you probably would have stopped having sex if this was the cause.
This is the type of thing that can be easily handled by your OBGYN. I suggest that you schedule an appointment today. Your OBGYN will perform a pelvic exam which may reveal where you are bleeding from. Your doctor may also recommend using a personal lubricant to prevent this from occurring in the future.