If it is food allergy or intolerance, it may be gluten or celiac disease. People with gluten problems can have severe stomach pain, vomiting, diarrhea or constipation. If it is celiac disease, the body's autoimmune system kicks in when gluten is eaten and starts attacking itself. You can get malnutrition and anemia from this.
It's just a thought (it's on my mind all the time right now since discovering this past summer I am celiac).
If you do suspect celiac or gluten intolerance and do want to do the blood testing or biopsy, do NOT stop eating the gluten until after the testing. Sometimes the testing is hit or miss as to accuracy, and you must be eating gluten for some of the tests to detect a problem. Enterolab does a stool test for which you do not have to be still eating gluten. But if you don't care about having testing done and want to just stop your child from eating gluten and see how she does, if she feels better after omitting gluten then she does have a problem. Just remember, it might be a few days before you notice a reaction ( food intolerances are often delayed reactions).
I can tell you from my experience, after having taken it out of my diet for more than 7 months now, if I slip up and eat even a crumb of gluten it gives me the worst burning pain, like no other pain I have ever felt (and I have felt excruciating pain before). Wanted to vomit from the pain.