Chronic unbearable stomach pains

My 11 year old sister has been having severe stomach pains for a month. She only gets them in the morning and at night. She is always constipated and has lack of appetite. We took her to 11 different doctors and none of them could figure out what's wrong with her. They took a blood test and a urine test and found nothing. Most of them say that she's just constipated. Could someone please help my figure out what's wrong with her?

My 10 year old daughter has started to experience the severe upper stomach pains. She cries and even vomits at times. It's been a few times a week around night time for the last three weeks. They think it is constipation with her too. We are still waiting on the blood results to come back but they are saying if that doesn't show anything they want to do a GI on her. I can't seem to find anything to ease the pain for the mean time. I don't know what to do for her. Anyone have any suggestions?