My daughter had very similar symptoms. They did the stool testing and blood tests and found nothing. A year later she was still sick, I asked for her to be tested for lactose intolerance and fructose malabsorption and she came back positive for both. We've made dietary changes and she no longer has any symptoms. You need to go to a special testing place to have them done, they are breath tests, and I don't think they are commonly suggested by paediatricians but it has changed my daughters life.