I am 15 pounds overweight, so why do I look so fat?

So I am 16 years old, and 15 pounds overweight according to my BMI. I am short, so 15 pounds is more apparent on me. The thing is, I swear to god I gain it all in like 2 places. First, my lower abs (like under the belly button). Two, my butt. Under my belly button I have a 'fat roll'. And it isn't even a pooch, it is bigger like an actual roll. It's gross! Why do I have a big roll there when I am only 15 pounds overweight? I don't get it, it is quite big and it makes me feel obese. I eat really healthy as of 6 months ago so that isn't the problem.I also have fat on my butt causing cellulite. I have okay legs, and my arms and face are thin, but I have this roll under my belly button. If I lose 15 pounds will all the fat come off my lower belly? I have a vacation in 2 weeks and although I have been eating extremely healthy, if I don't lose the under belly button roll I will not be wearing a two piece bathing suit. Can someone help??