What Causes Dark Spot On The Corners Of Mouth?

I have dark spot on the corners of my mouth since about a year now. It happened very quickly - maybe allergic reaction to something or some yeast/fungal infection. Right now I'm using Kojic acid with Hydroval. But I don't have much hope in this. I'm female in 50's, 130 lbs, 5ft 3in and good health.