I don't know if I've had exactly what you experience, but "outrageous stomach pain" does describe what happens with me every once in a while. Sometimes related to something I've taken orally, sometimes I can't find any connection.
The times when I can't find any connection are terrible. I know what gas feels like, I know what diahrrea cramps feel like, but this is so severe I'm non-stop yelling my head off and rolling on the floor, writhing uncontrollably with brief periods of explosive diahrrea and then becomming nauseous and vommiting from all the pain and sudden loss of fluids. This happens out of the blue with no change in diet or meds.
When it is related to something I take orally, there is a couple days' delay. I begin to burp a very specific taste/smell that is the most putrid thing I've ever experienced. If I only consumed a little bit of the offending item and immediately stop whatever is causing the problem, it'll just cause minor pain for a few more days and a lot of discomfort before it goes away. But if I consumed a large amount of the offending item and/or continue to take it, it goes full-blown and the pain is....bad enough to get an ambulance. It feels like my stomach is digesting itself, truly the worst pain I've ever experienced, bar none---and I've experienced a lot. After about an hour of this pain I'll begin dry heaving and then vommitting so violently that once it cause a minor tear somewhere in my throat.
I decribe this because the first time it happened was when I began using oral morphine for pain management. It took two ER trips in an ambulance and on the way to the ER a third time in a car before we figured out it was the morphine that was causing the problem......IV morphine is just fine, but my GI freaks out in this way, that's why it wasn't helping. I cautiously tried other strong pain meds with the same reaction, turns out they're all morphine derivatives.
Now get this: rice causes the exact same problem. Don't ask me how or why, I have no clue. White rice, brown rice, fermented and soaked brown rice, organic, it doesn't matter, if I eat some, two days later I'll have the burping that distinctinve putrid odor and the beginnings of feeling like my stomach is digesting itself. When I first became severe with CFS rice was one of the few thigns I could eat, so that doesn't make sense to me.
Anyway, based on my expereince I suspect the pain meds might possibly be to blame, though I know that sounds perposterous, that's the way it was with me (with some foods as well). The tricky thing was the reaction was delayed by a few days so it was hard to figure it out at first. I, too, have had tons of tests on my GI, and though I do have celiac and such that shouldn't affect this.
Sorry I can't be of more help!