What Causes Vomiting And Nausea?

I have a 14-year-old daughter who hasn t been able to keep hardly any food down for a few weeks. She doesn t have the flu. She only feels nauseous and throws up after she s eaten. She can pretty much only keep down clear watery fluids (tea). We ve tried pedialyte, juice, toast, applesauce, smoothie. Our doctor is ordering an ultrasound, but I ve been doing a lot of research that says other people have had these same symptoms and even GI specialists have problems diagnosing or helping them.