My dd was diagnosed with celiac last year. I know your dd is small that is common in kids with Celiac as they do not use the nutrients as well. My dd would out eat me and her brother and she was 105 pounds less then a year after diagnose she feel as better and weighs 130+.
THIS^^^^ At least insist the doctor do a blood test for it. My oldest daughter has it and is one of the main causes of digestive issues. It also runs with Allergies like Amy said. My oldest daughter is allergic to everything. Most doctors right it off as Irritable Bowl Syndrome (IBS), so you must INSIST on the test. Good Luck and I hope you find the cause.