Have you had a CBC? Anything show up?
I would recommend you getting a Comprehensive GI Stool anaylisis from Metametrix: Here it is. Pay the $$, it's worth it, otherwise you'll just be guessing. Best test there is.
by the way, I just eliminated oatmeal from my diet, and I'm a lot better. I've eaten it my whole life, but my digestion seemed to get worse and worse. Every afternoon, hours after lunch, painful bloat and locked up gas. It got so horrible. some mentioned to me to stop the oats. I've never been allergic to gluten or
any grains either. But who knows if that would show up on a test now. It's definatley a delayed sensitivity, which are tricky to diagnose, but I had a delayed sensitivity IGG test, and all was fine with oats and all other nggrains. You could think about getting some allergy testing as well. Do IGG and IGE, and some kind of delayed sensitivity test.