Why did i gain weight? please help?

okay so im 16 and i have a very strong athletic body. every part of my body is muscular and i am very active. About 1 and 1/2 year ago (the start of high school0, i started to put on weight and went up from a 4- 6 to a 8-10. my legs now have cellulite around then and my stomach is starting to get a little flabby. my arms now have a slight layer of fat around them. i don't know what i did wrong! i really didn't change my diet or exercises and now i have stretch marks on my inner thighs! now i am seriously trying to lose some of the extra weight and be muscular again. i know i cant be stick thin because of my athletic body type. i run 2 miles every other day and eat a very healthy diet of fruits, veggies, nuts, low fat dairy products, and lean meats. i let myself have two little exceptions a day like ice cream or if its a special occasion i can splurge. and honestly this is not that different from my diet before i gained the weight. so where did the extra pockets of fat come from and how can i get ride of it? everything i am already doing is not working. i don't just want a quick fix like every one else. i am up for the challenge to do whatever it takes for how long it takes. i just am totally lost on what to do.