I have been experiencing this same problem on my knee for several months now. It feels lika a sunburn but the skin looks absolutely normal except for a slight hint of discoloration in that area if you look closely. It's just a spot about the size of a 50 cent piece on my left knee just above the kneecap. I'd really like to know what it is and if it's something I should be worried about. Most of the time it feels normal, and then out of the blue it will sting for a few seconds.