Is it a type of Allergy or Something Else?

I have a red Itchy rash on my stomach, neck, ears, chest and back.  I originally had the same rash on my legs and chest, then I went to the dr and he gave me prednisone.  Which did nothing but suppress the rash.  Now a month later the prednisone is gone and the rash came back.  Now its not on my chest but on my stomach, back, neck, ears, butt, and legs.  I haven't changed anything or ate anything strange.  I know my allergies.  I even switched to a hypo-allergenic soap to make my dermatologist happy.  Still nothing.  Its getting a little annoying having a rash go away as long as I'm on meds but nobody can tell me whats causing it.  I'm 30 years old.  I never had a rash like this until this past november.  Aside from Diaper rash,  I don't remember ever having a rash period.

Any idea what it could be?