You know what, I think you're growing and it's probably a small layer of fat, perfectly natural at your age, I mean I'm a teenager myself and I've got enough podge to be getting on with!! Try some gentle stomach exercises if you're REALLY uncomfortable, I would recommend sit-ups, I go to the gym regularly and my stomach has become more toned since I started sit-ups. I would aim for approx 50 each time, start off doing 10-15 and take a 10 second break, then do the next 10 etc, and you'll gradually find it easier. Good luck!!