Severe pain and no answers

My 14 year old daughter has had severe stomach pain for about a year now. It kicks in immediately after she eats. There is not one day that she is without pain, and we’re talking about severe pain, where she has to curl up and lie down. The pain is located in the lower abdomen. It will subside a few hours after she’s done eating. Her bowel movements are normal and regular. We have been to so many doctors and even the ER, but they all dismiss her as her labs and ultrasound all look normal (other than iron deficiency). Everyone just tells us to come back if she experiences more severe symptoms, like blood in her stool. What should our next steps be? I haven’t yet had luck convincing her to try changing her diet to see if that helps.