What Could Often Vomiting Post, A Meal Suggest?

my daughter has severe vomting after eating. she is fine until several hours after a meal. then she vomits most of the contents in her stomach. this usually leads to dehydration that sometimes requires a trip to the ER. the doctors don't know why this is happening. she is fine though after eating the fruits and vegetables from our friend's farm. most anything else leads to vomiting.