It could from be a few reasons...
1. If this only started after you began taking the pill, you could be bleeding because of the pill, not because of having sex. Different methods of contraception
have different side effects, especially with oral methods. With the pill, you can easily "spot" at random times of the month.
2. You may be having sex that is too rough.
3. If you're a "small" girl, he may simply be too big for you, as that area is very sensitive and can easily be bruised or torn slightly.
I'd say to first decide if maybe the sex is too rough, and to try and take it a little softer. If you are still bleeding, wait another few weeks, and then ask your doctor for a different brand of pills, if this is still happening. If after you change pills you still bleed, you should probably decide to talk with your doctor about it.