What Causes Rash On The Neck And Stomach?

I am an almost 17 year old male, 5'10 & 140 lbs. I have a red itchy rash that moves around my body such as my back, neck, stomach, shoulders, hands, palms, and tops of thighs near the groin area. I haven't been on any new medications, changed my diet, or anything. This just happened sporadically two nights ago. Any ideas of what could be wrong?