What you describe with painful attacks involving vomiting sounds like stomach migraine. This is an actual condition that now has diagnostic criteria (according to the Rome protocol). For people who get these attacks regular migraine medicine can stop the attacks. (For example you can use the type you squirt up your nose.) Migraine meds are not recommended during pregnancy but maybe it could help you in your life in general in case this is something your doctors have missed. I think this condition is still fairly unknown.
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Maybe this could be helpful? (Stomach migraine)
(also pregnant at the moment)
