When my boyfriend and i have sex, i get a sharp pain in the front of my lower abs. it happens when i am on top only. what is this sharp pain?

Trauma: When he is having sex with you, you are causing trauma to your internal organs, and your positioning sounds like this is aggravating it. Try different positions and nsaids like ibuprofen, but if not better, be seen by your doctor.

Pain during sex: If the pain you are having is only then perhaps is not that bad. So more specific: if pain is only with deep thrust, there is no vaginal bleeding, the pain is not getting worse over time, is not associated with nausea, vomiting, probably relates to a stretching of your uterosacral ligaments which can be painfull. Yet, if pain is getting worse over time, see your doc.