Flab isn't just the 2 extra pound of fat you can see, it's probably closer to 5-10lbs of extra fat on your entire body. The part you see is just where your body concentrated the storage. Your body does not evenly distribute fat. It has parts of your body where it prefers to store extra fat before storing it elsewhere. The stomach is one of these areas.
You say you can see your abs? Abs are EXTREMELY hard to define, and for some genetics, maybe even impossible (I can't say that for sure though, however I can assure you that the abs are, generally speaking, the hardest area of your body to develop visible definition of the muscle. Not only do you need large muscles, but you need a VERY low percentage of body fat so the muscle isn't hidden under the fat padding.
You definitely don't sound overweight. It just sounds to me like your body stored some fat in your gut rather than spreading it out all over the body. A little pudge of flab at the bottom could be a number of things, especially at your age. Hormones, growth spurts, anxiety, these could all cause your body to store extra fat here and there.
When you do hit your growth spurt, you'll probably notice yourself getting a little pudgier. Slight accumulations of fat or weight gain is perfectly normal at your age and is just part of growing into a new body.
This doesn't mean you shouldn't be exercising though. Exercising will definitely help to keep your overall weight down, and is most beneficial when you're still going through puberty, so now's the best time to start.
Just eat well, be smart, realize that your body is only going to reflect how you treat it. If you know you're healthy, and you know that your body can perform well athletically, you have all you need to find complete confidence in yourself, regardless of whether or not you have a little patch of growth weight showing every once in awhile.
