For the past few months my daughter has been very ill

My daughter is 15 yrs old. For the past few months my daughter has been very ill. She started with feeling weak and dizzy. Then a few weeks later she started feelin weak, dizzy, headaches, fatigue, and her body would shake from head to toe for about a minute or 2 no longer. She would be talking to you but not making any sense. She would pass out or black out. She couldn't remember anything you ask her, or even what she would say. My daughter has been to the doctor the ran gluclose test (4hr), the ran a sleep deprived, and MRI of the brain and a heart monitor for 2 weeks. The gluclose test showed she low blood sugar but not that low. Her sleep deprived showed normal. Her MRI showed sinus infection, but no lesions or tumors. Her heart monitor test has not come back. My question is does anyone really no what is wrong with my child. She has missed 21 days of school due to this. And my child is A,B student. Someone give me something to go on. HELP