Pain in stomach during sex? 21 weeks.

For the past month every time I get on top during intercourse I get a sharp stabbing pain in my stomach to the point where I have to stop. I'll stop for a minute and then go again and the sharp pain comes back in my stomach. It feels like the baby hurting me but I don't know. It hasn't stopped for the past 3 or so weeks so I try not to get on top anymore but sadly that's the only way I can orgasm. Ugh. Have you ladies had this happen? Should I call my doctor? The pain is pretty severe but since I know what causes it should I just refrain from going on top? Feeling a little worried..