Teenage weight? :( BMI, self-image issues?

Hi i'm 13 and i'm gonna be 14 in february.... I'm really worried because i somehow gained 2 kg in one week without changing my diet or doing extremes in anything?? Last year in July i was 57kg then this year the scales crept up to 62kg then to 64kg and then within a week to 66kg?!? I am 167cm. I do sport 3-4 days a week and I'm not old enough to go to a gym plus my sport isn't in season yet. :( Could you please work out my BMI to see if i'm healthy? I'm getting really anxious and body conscious.... i don't know if i'm the only one? I have an hour gl figure but I have muscle because i used to do tons of swimming. i don't know what to do because my mum buys biscuits and stuff for school lunches (NOT HUGE AMOUNTS! LIKE ANY OTHER MUM IT'S IN PORTIONS) i have an extremely slow metabolism and i feel lke my body is gaining and gaining while im not watching while i'm not even eating more/less.... please tell me if i'm normal. i feel so useless.