I Noticed A Small, Round, Dark Spot The Size Of

I noticed a small, round, dark spot the size of an average mole, but flat, seemingly under the surface on the very tip of my big toe on my left foot. I noticed this about a month or so ago and it hasn't gone away only increased slightly in size and depth of color. There is no pain at all associated to this in any way. Thoughts? I do not have diabetes.