What Causes A Black Spot In The Navel?

I am 19 years old and I noticed a black spot in my belly button near the navel but it was so small I didnt think anything of it. It has been there for awhile now and I have no skin problems or any symptons of any sort. It has grown a little larger and I thought it was because I had gained some weight. I have not paid attention to it until the other day when I got my belly button pierced and saw that it is still there and larger.