I think it has something to do with the soluble or insoluble fiber. Sometimes our guts cant handle one of them. I heard that from a dietary consultant when I worked at a hospital because I had that problem.
I've been sick from honey nut Cheerios, quacker oatmeal squares, and oatmeal, raisin bran, and wheat chex that I can remember. I think I handle rice crispies ok.
What kind of cereal? I have the same problem with oatmeal and cereals like cheerios, honey nut cheerios, fruity pebbles, lucky charms. I am fine with honey combs and chocolate rice crispies. (rice crispies are gluten free) I think it has something to do with some sugars and or certain fibers in them. I don't have celiacs or a gluten disorder that I know of eaither.