For a girl to get a stomach ache after anal sex? 
yeah, depends how thick your cock is and how tight she is... your just pumping air in her ass....
some girls i ass fucked got stomach aches, but the funniest was this cute 19 yr old girl, damn she was sooo hotttt.... she was a freak too, her ass was so tight just putting my middle finger in it streched it out... anyways, after 20 mins of warming her up, i ass fucked her till i came... 2-3 minutes later she couldnt stop farting.
true story, she was sooo ashamed, but funny as hell