My dd has been diagnosed (finally! after 5 years) with Gastroparesis. It is not a result of a milk allergy or anything like that, but is a stomach condition and a condition related to the Vagus nerve. The Vagus nerve can be damaged after a trauma (car accident) or injury or from several other causes. Sometimes they just don't know what causes it.
With Gastroparesis, the stomach doesn't get the message to start breaking down food, and so basically food sits too long in the stomach. The burping and hiccuping can be horrible and painful. There can be abdominal pain, nausea, vomiting, a bad taste or smell in the mouth, and constipation. Symptoms vary widely.
My dd has just started taking a "motility agent", a medication that binds to the muscles in the stomach and gets the stomach doing what it is supposed to. There are tests for it, but my dd hasn't had hers yet. It's coming up soon. But since she has every symptom of it, and several of the causes of it, they're already starting treating her for it.
A GI specialist will know about Gastroparesis and can help you. In the meantime, here are some things you can try. Have her eat smaller meals. Avoid raw vegetables like carrots and celery and things that are harder and therefore make the stomach work harder. Smoothies, protein shakes and liquid nutrition are fairly easy to digest even with Gastroparesis. Other things to eat are: soft foods like scrambled eggs, cooked vegetables, pureed soups, etc.
I hope this will help you!