What Causes Rashes?

i have a rash going along my collar bone. it started at the small of my back and i thought it was just acne but then it worked up to my shoulders, i still thought it was acne. Now it goes all the way from the base of the back of my neck and over my shoulder and lays on my collar bone. Its a bunch of tiny red bumps. They itch every once in a while but its not terribly bad. Its been there for 3 days now and its not getting better. Im also afraid it might move down my chest. Any ideas as to what i should do and/or what this is?