What Causes Itchy Skin Rashes On One Side Of Body?

I get skin rashes on one side of my body which itches like crazy , the more you itch the bigger they become. They last for about 2 weeks then leave a very dark spot on the skin which goes away after 1 - 2 months. This occurs once or twice per year, sometimes as one side clears the other side is affected. They start as small bumps or rash then increase in size, there is a raised bump i the middle and a red rash around it, it usaull occurs on the lower half of my body especially the legs. Does not affect my genital area