I have a somewhat itchy rash across my upper back, and more mildly across my upper chest, what could have caused it? what is it

I have had this rash for 2-3 days, as far as I know it is not an allergic reaction to food or anything topical.  It itches, but not all over, or all of the time, it's mildly itchy.  There is a lesser rash on my chest that seemed to go away and reappear.