What will happen if i eat 300 calories a day or not enough?

i just got braces and they are extremely painful (never under estimate how bad they can be ;) ) and painkillers don't work so i've only been able to eat about 300 calories as it's a very strong pain. i know this will get better, but when i first got them and had no pain, i couldn't eat anything either as i couldn't chew without all my food getting stuck in my braces and i have a piece of plastic in the roof of my mouth that makes it harder. i'm also very picky with food so there isn't very much i want to eat that i can eat. what sort of effects will i get like putting on all the fat i eat?