Dizziness, shaking, fatigue...

Last year during gym class, i fainted. Ever since then i've been dizzy everyday. My mother and the school nurse think it's because I do not eat enough, so I've been eating and drinking more, but it doesn't help. When I sing, dance, run, or stand up really fast, i get really dizzy and sometimes I fall because I can't move. Once I was running and I had to stop and I couldn't move or speak i felt like i was going to throw up but i didn't. i'm in constant pain and i can't concentrate anymore. Also I sing a lot, everyday but something happened and my voice doesn't work well anymore. It hurts to sing and my throat gets dry and i get dizzy. What do i do?!!