Can you help me figure out why im fainting?

it doesnt happen everyday its random. i do eat alot n sometimes i make myself eat because if i dont eat i lose weight like it aint nothin. i dont actually pass out tho i get all of the symptoms and by the time i sit down i have to eat something like a candy bar before i can get up again i almost fainted 3 times in a row one day until i ate a candy bar then i felt great the rest of the day. i think its caused from low blood sugar but im not sure every time i go to the doctor for somethin they always say o...well we dk whats wrong with you so i jus thot id ask some other people before i went to the doctor.