Have you ruled out all other possible stomach, intestine, bowel disorders? Sounds like it could also be a nerve problem, or a neurological problem, maybe even anxiety. Obviously you don't have typical symptoms, so the doctors would have to do lots of testing, and actually think to figure out what is wrong with you, and they should because you are obviously suffering from something. Don't let them just look at you like you are just nuts, if that doctor doesn't help you find a new one, that's what I would do. Good Luck I hope you get it figured out, and feel better soon.
Also if you haven't already you might want to post this question in the IBS forum as well.
Have you been tested for allergies? If so you might want to, if for nothing else to rule it out.