I nearly passed out today!?!

I nearly passed out today!?

We were having graduation practice today, and yes it was hot! (90 degrees) I was feeling dizzy about 5 min. before we started practice. During the middle of a song i felt faint. I didn't have control of my body, and i started swaying. Everyting turned very bright, then completley dark. The music sounded far away. I was about to pass out, when a teacher ran up to me and shook we voilently, and yelled in my ear. What could have caused this?

A reaction to the heat or possibly low blood sugar (or a combination of the two).