Don't worry, it's perfectly natural. Before puberty, the abdominal region of both males and females is covered with very fine hair. During and after puberty, the skin of the abdomen begins to produce coarser, longer and more pigmented hair.
You must be at your puberty now, when you start to grow coarser and longer hair on some areas of your anatomy. Abdominal hair is usually found in males. If you are female, it could also happen but watch out for hormonal imbalance. You could also be a normal female and have a few coarse abdominal hairs, but not as many or as long as a male's.