I have a light black line under my nail on my left thumb for about 3-4 YEARS now. It doesn't hurt or anything and I've always been very healthy, although my skin has never been the "healthiest". I'm in my mid/late teens and asian in race. 

It runs from top of the nail to the bottom of the nail and is about 2-3 mm wide. It grows out as my nail grows and it's just been there. But recently, it seems like it has gotten wider so I'm worried that it's some skin cancer. I checked all my moles on my body and they are all normal. 

Any ideas? Thanks.