What Causes Rashes On The Buttocks And Thighs?

I have been experiencing a rash on buttocks, back of thighs that acts like a sunburn. I have a small rash on buttocks that comes and goes. This is made worse by showering. prolonged sitting, clothes rubbing. Sensitivity on low back to tags on my clothes. I feel like my back is now beginning to feel hot as well. Redness goes away but the burning (like a sunburn) often continues. I have chills on and off. I am on 1 gram of valtrex a day as a suppression treatment for the cold sore simplex, although the cold sore when it appears does it appear on my left buttocks. I have had the blood test to diagnosis simplex 1(?) did not react to the genital simplex. The rash doesn t have sores of any kind. No blisters. Almost appears like a heat rash. I am 52 year female who is otherwise in good health. I do take a statin and have for several years. Preventive for a family history of heart disease.