Hello~There could be many reasons. How old are you, it could be due to fluctuations in hormones. Have you had your thyroid tested, having an over-active thyroid can cause extreme inner heat. Anxiety and extreme stress can cause you to feel hot, but normally folks do not pass out with it.
I am sorry I can't be of more help, not knowing any more info, I really can't give a more direct answer. I would suggest seeing your GP however if this continues.