Take her back to the doctor.
Watch her diet, many cannot have onions, garlic, msg, artificial preservatives, nitrites, nitrates, caffeine, milk, wheat
Make sure she wears sunglasses or if she wears glasses get her Transitions.
Make sure she is eating 3-5 meals a day, little meals, and healthy, even just a granola bar or apple in the midmorning and right after school. No skipping breakfast.
Does she also get nauseous? Lights bother her, noises, smells? Poor thing. My son's were triggered by changes in light, so going into an aquarium was a trigger, tv in a dark room, going from inside to outside in the sunshine. My friend's daughter's are triggered by foods.
Hope she starts feeling better very soon