itchy rib rash

I have a really super itchy rash that covers my entire ribcage on both sides and a little in between. (raised but not blistered) The doctor I saw said it was a contact rash (like poison ivy but it's not poison ivy) and gave me a steroid cream. Thing is, I have had no contact with anything and if it were that type, it should be spreading but it is only on the ribs. It itches so bad and of course is painful from scratching. There is one small section of the right rib that is very painful to the touch (like bad bruise) which the doctor said was also from scratching but why just one spot would feel that way and not everywhere I am scratching, I do not know. I think there is more to it but can't find any info that describes it. Anyone have a clue? PS- I have not changes detergents, deoderants, bodywash etc (and again, it's only on the ribs). Thanks.