What Causes Round Spot With Stinging Sensation On Forearm?

Hi I noticed about probably 3 weeks a round spot on my forearm that I tough was maybe a bug bite or something. It itched and kind of stung at first. It has turned dark purple with a red ring around it and kind of gets a white flakey dry layer over it that I tend to pick at cuz it annoys me. but it won t go away. and it is slightly raised from the skin. What could it be?