Well if your physically fit in all areas besides your stomach then it just might be your genetics. Is the rest of your family skinny but have some flab in that area? People always have those certain areas where they just can't loose the weight where they want to so if exercise, dieting etc. doesn't work I would do some liposuction but that's only if it really bothers you. Plus your only 15 you'll grow into your body more so who knows it might change.