First, if I found myself in your shoes I would take her back to the hospital (If you haven't been to Children's yet -go, and if you've been- go back) I would insist on another examination, and that the staff acknowledge your daughter's suffering and help her to find some relief. I encourage you to be an uncompromising advocate for your daughter, until they *DO* make a determination as to what could be troubling her. Primarily, that is my humble advice.
In regards to what could be the cause for her pain I imagine that some intestinal distress is possible maybe an impaction? That could produce those symptoms...
Maybe an ulcer...
Maybe even a strained muscle, those caused me intense pain in the gut when I was a child, usually after swimming, though.
That is all the possibilities I can think of right now that could fit the symus oms. BUT (this is my big but!) I think it is crucial that she be reexamined now that you see it doesn't seem to be letting up on her. Poor thing. You are an excellent mommy, don't let medical staff ignore or write off your daughter's needs. I wish you all the luck in getting to the root of the problem and making your sweet girl all better. I hope you will be sure to share once you two get some satisfaction.