I think there's something wrong with me.. help?

I dont know what it could be.. I'm 16 and I'm totally preoccupied with weight, most of my questions on here are about weight. If I see a girl i always try to guess what she weighs and it's always on my mind. I'm obsessed with numbers and counting and converting, I know my weight in pounds, stones and kilograms and I'm constantly comparing myself to other people. My friends say I'm thin but I see myself as fat and flabby and I can spend hours prodding and squeezing my fat. Several times I've been on pro ana sites and I have seriously considered stopping eating. If I look at images of underweight girls, they dont look that thin to me, they look perfect and what i wanna be. I've been bullied in the past and don't feel good about myself and I'm lacking in confidence and self-esteem. I also am a bit OCD about things like I can only use certain shapes of cutlery and eveything has to be ordered. Whats wrong with me?