Blood after sex sometimes.

Whenever me and my boyfriend have some deep, intense sex (nothing other than just a bit more heated intercourse) I sometimes have bleeding from my cervical area like a period.

It usually doesn't hurt, but a few times I've cramped when it happens. We have been together over 2 years and it's only happened probabaly.... 15-20 times since we have started having sex (2 years ago).

It always clears up and is never really anything to go to the doctor over. The amount of blood after this type of sex even at its worst is never more blood than what a normal meduim period would be for me.

Ideas?