I am sorry to hear your daughter is feeling so ill. I am 15 and I have very similar symptoms to your daughter and I also have no answers. I was diagnosed with celiac disease when I was 13 years old, but it didn’t stop the GERD, diarrhea, and tummy pain that has been plaguing me for quite some times. I got a colonscopy and an endoscopy too, but they found nothing. Eventually my GI and physician pretty much just diagnosed me with IBS (irritable bowel syndrome) and told me I would just have to deal with it. It’s been quite frustrating too because I am 5’4 but I weigh 85 pounds, so I am very thin. I guess I am still searching for answers.
So far the only thing that has given me some relief is massage therapy and yoga.