K.H.
I would look at the brand and how it's processed to help you determine what the cause of the problem was. Does it have preservatives or flow agents? The only thing that caused that type of reaction in my daughter was acidic foods like apricots and peaches. Her system just wasn't ready for the acid. While we hate to experiment with our kids, we kind of have to when we come from families with allergies. I think you're doing the right thing introducing one new thing at a time. Oatmeal is a not wheat grain and it would be a shame if she couldn't eat it. I'm thinking it's the brand since the baby oatmeal wasn't a problem. If you can't get another brand, write home and have grandma or grandpa send you samples of different brands. Start with an organic one.
Good luck,
K.