Why do i have a flabby belly?

im 13 years old. i usually eat pretty healthy i mean i eat a lot of chocolate when it craves me..oh and how can i stop the need for sweets? anyway i do karate 3 times a week i do get my period(if that has anything to do with this) but when i look at my BMI it says that i am normal weight but how come i have like a bulge at my belly....not huge but like a little bump kinda thing that when i wear a bikini it looks like im pregnant and im not because i am not ually active. but i heard that running in sweatpants and a jacket will help lose weight..is this true? and any ideas on how to loose the weight so i dont look bulgy in a bikini for summer?