Can't get rid of belly fat?!?

im a 16 female and in ok shape, not skinny and not chubby. i am kinda skinny in my rib area, but for some strange reason there's like a little pouch of chub in the belly-button/lower half of my stomach. i dont know how to get rid of it, i have tried everything, dieting, exercising, etc. could it be a tumor in my stomach? please give my suggestions.