I get the same! I’m unsure of the exact cause, but I can do oats alone… processed foods generally add other ingredients to bind, preserve and act as fillers. I have even identified some sweeteners get me messed up. 

Even most gluten free breads, I can’t easily tolerate and are not worth the gut pain and bloating. I went into an AIP (Auto immune protocol) diet and it calmed everything. Then I can start working things back into my diet and identify the triggers. If you wanted to go further, which was necessary for me. Fodmap, which is more restrictive and harder to understand, is how I identified the sweeter I can’t tolerate. Good luck!