Suggest Treatment For Rash On The Buttocks

After the birth of my first child, of which was a Cesearian sectio, I started getting a rash on my buttocks where the shot of dememrol was given to me. My OB said it was symptomatic of an allergy. I still experience this rash now and then, it starts out red and blotchy, and I put caladryl on it, it raises to like hives, itching and then the caladryl dries it out, then it s gone. The location changes, and seems to occurr when I am stressed. Could this be Shingles?