Darker skin around mouth?

I have very fair skin, and it is all even except the around under my nostrils all the way down to my chin is darker than the rest of my face. It looks like a shadow or a slight dirt and I can't seem to get rid of it. I've been using Palmer's skin evening products for about 2 months now, but it doesn't seem to be working. Anyone know what this is or how to lighten the skin - without just covering it up with makeup?