Please help me? Lots of questions! ?

I'm 14 years old, Female and I weigh 7 stone. My height is 5'2. Over the past 4 months I've become obsessed with calorie counting, I won't eat meat, sweets, chocolate, potatoes or anything that may put me over my limit. My mum thought it was just a fad, but even I admit now it's turned into an obsession :( On school days I try and stick to as low calorie intake as I can usually 300-400 for the day and on the weekends I'm stupid and greedy and consume 500 on average. I feel tired and Ill all the time and sometimes feel very dizzy and faint, I've never fainted before though, please help me and tell me what I should do, I'm terrified of gaining weight, like absolutely petrified that if I over eat for one day I'll put on weight. It's ruining my life :'(