Why am I gaining weight if I started exercising and I eat within the calorie range for my age?

I'm trying to lose fat and I started exercising but I just seem to be gaining weight and my stomach looks a bit bigger. I eat about 1700 calories and burn 250 through exercise, any reason why i'm gaining weight? By the way i'm 15, 5 feet 8.5 inches, 125 pounds, not that much muscle and some fat spreaded on my body.