Why Do I Gain Weight So Easily ?!?

Im 160 Pounds , 13 Yeard Old , 5'0" Tall . I've Been Chubby Since 1st Grade . But Now , I Feel OBESE ! Last Week I Was 145 , Five Days Later 158 , Today 160 . I Dont Exercise , But In P.E I Walk &&' Run . I Dont Eat That Much , I Eat Breakfast , When I Get Home &&' An Atkins Snack At Night . But I Still Seem To Gain ALOT Of Weight , I Dont Understand ? SOMEONE PLEASE HELP ME !