Although from your symptoms the first possibility that came to my mind is of shingles but since you have had it prior also, so other possibilities need to be ruled out. It can be due to intertrigo also which is an itchy rash and at the same time is sore.
My sincere advice would be to consult a dermatologist and get it evaluated. Treatment is diagnosis specific. It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Wendy, It is Jan 2011 and I have my second rash (2 weeks free) exactly as you describe it. I had shingles back in July. My doctor said it was a herpes rash and no treatment helped but it is back again so any more ideas!