C.S.
It sounds like an allergy to me. My daughter is allergic to corn and unfortunately, I was not as "in-tune" and needed an allergist to tell me.
To begin with, corn is a difficult food to digest. However, my daughter tested positive with an allergy to it and it sounds like you daughter is too.
Once I pulled my daughter off of the corn, she was much better. Label reading is important because you will find corn in nearly every cereal found in your common grocery store and as well in so many of our foods in different forms. You will probably find it easier to shop in whole foods or trader joe's for certain items.