What Causes Constant Pain In Stomach?

My 13 year old dauaghter has been experiencing stomach pain for approximately 2 months. The pain is localized- from her navel area to the right side. At first the pain seemed to be whenever she ate,but since 3-4 weeks, it has been constant. She is often in severe pain,especially at night. She says it is stabbing pain and sometimes it feels burning. She has seen a pediatric GI. She has had numerous tests- CT scan,ultrasounds (ovaries and gallbladder), colonoscopy, endpscopy, and HIDA scan. All the tests have been negative. She has been taking Miralax daily. I am very concerned as the pain is constant and often will wake her at night. Any input.suggestions are appreciated.