You know what does it to me...Whey protein.

Since the age of 17 through probably my late 20's I lifted periodically and always consumed whey protein. So I spent at least 10+ years thinking I had IBS. Road trips were especially bad because I never had time to eat so I'd always down protein shakes because I thought that was a good idea. Turns out when I eat whey protein, any time between 5 hours and up to a day later, I will feel an explosive rumble near my sphincter with an immediate need to empty that rumble.

So anyway, maybe take it back to the basics to do some investigating.

Start with the ABSOLUTE basics...like seriously start with just gluten-free, non-processed foods only, I would have to guess oatmeal or gluten free oatmeal bread and natural jam. You'll probably be a bit hungry your first day or two, but as you start adding things in day by day, you might just find out what's bothering your stomach. Of course it could just be your stomach, but it could be something you're totally unaware of.