I get this too. When I went to the doctor's and had my pap done I got stomach pains when my cervix was touched. Any deep positions (well for me its upright on top) I get the stomach pains cause he is hitting my cervix.
Since you only get them in certain positions, i'm guessing you have the same problem.

The same thing happens to me but only if we do dog stiyle my pain can get realy bad an last for a few hours so if we do do it he dosent go as hard an deep

Last night my bf and i were having sex, and it was all very deep positions, and he bumps my cervix which is uncomfortable to a mild pain for me, but quite pleasurable for him, (he says it "tickles" the end of his dick) so I allow it. But last night for the first time while we were doing it doggie, I started to feel incredible abdominal pain. Which I had to stop immediately it was so bad. (Of course we finished). But my abdomen is still quite tender this morning. I had to take 2 Advil's to get to sleep, and all they did was dull the pain. I'm guessing that the deeper than usual thrusting either has irritated the uterus wall, or I pulled something.