What Causes Persistent Pain In Lower Abdomen?

Hi I have a daughter who will soon be 16, she has lower abdomen pain almost every day, she has been going back and forth to the doctor because of this pain for about two years and they cannot find the problem she have had x ray, ultrasounds and CT scan done and everything came back good. I am very concerned and don't no what else to do it is very upsetting to see her in pain all the time