There is a period of trial and error to determine what foods you can tolerate. I agree with the elimination diet. You can do everything at once, or one group at a time. The common things:
Regular milk (even though other dairy causes no issue - but some tolerate very little dairy at all) - this may includ whey protein, too.
Gluten - Beware especially of whole wheat bread, especially, for they add extra gluten to it. I've gone back to Italian bread for my daily toast, and have no problem at all now.
Yogurt - eat it! I notice a distinct difference when I've gone too long without enjoying any yogurt. The live cultures are a great probiotic benefit.
Some veggies or legumes may affect you now differently than other days.
Artificial Sweeteners - if you're substituting to kick a sugar habit, this can wreak havoc, from baked goods, to hard candy to gum.