E.S.
My daughters both have food allergies and one of the reactions to foods they were allergic to when they were little was a rash around the mouth and either on their back or stomach. Write down any unusual behavior like crankiness,gas, or unusual bowel movement after the oatmeal. Call your doctor and find out what they think. It sounds like an allergic reaction to me. Keep her away from all things oatmeal related. Be very careful in case it is a food allergy and chances are she may be allergic to other foods as well.