Lip discoloration

I'm really not sure what it is but basically, there is like a really dark line on my lower lip at the part where the pink color of the lip meets the fleshy pink of the inside of the lip (going horizontally). It makes my bottom lip look half super dark (brownish/black) and half normal color. Normally my lips are really pink so it's starting to make me worry. I'm just assuming it might be a fungus, but I don't know how I could have gotten it or what to do. The color of the discoloration is significantly darker than my upper lip