Could be a cardio problem too, like POTS. Search on this site for Lynn63. She's posted about her daughter, about your age, with similar symptoms. They finally got a diagnosis and it was POTS. Mayber her posts will help you. Has ALL your blood work come back normal???
Ms. JRoger,
I recommend you request a referral to a Neurologist, Children's Hospital, or at least ask for a head CT.
Especially if all Gastrointestinal issues are ruled out!