Why am i putting on weight when i hardly eat?

Right, i have always been a fat kid, but then i lost it all through starvation and got down to 8st 13 and for a 5ft 8 girl that's fine. But recently ive shot back up to 9st 12? its adding on mainly around my stomach and hips and its starting to show, but whats baffling me is i hardly eat a thing? im a vegetarian so meats not on my agenda, usually in a day, i'll have cereal and then beans on toast or a quorn burger, that fills me, but now even eating so little im piling on the weight, i need help because im getting to the point where i dont want to eat at all again, because whatever i do eat puts weight on, its baffling me because i go to gym twice a week, i walk everywhere? yet this weight will not go away! Im not pregnant, i am quite stressed because of exams but they're out the way now, so help me please, im at my whits ends, clothes are beginning not to fit, my face is filling out arghhhhhhhhhhhh