My 5 Year Old Daughter Has The Episodes Of Throwing

My 5 year old daughter has the episodes of throwing up first thing in the morning until about noon and then it stops and she is back to normal like nothing had ever happend. She does complain about stomach hurting during the episodes. There is no fever and no other symptom. Her doctor can not figure out what's wrong with her and this has been going on for 3 years now. She may have this episode once or twice every other month. When she vomits sometimes it's clear but foamy and other times it's neon yellow and foamy . Please help me to understand what's going on with her. Thanks!