Suggest Remedy For Itchy Rashes

Hi: I have an itchy rash that erupts from time to time, it did originally start at the upper section of my spine in the lower lumber area, then over a few years, slowly progressed downwards until now at which time its located at the tailbone. It will probably move slowly back up the lower lumber spine. It arrives usually when I am tired and stressed, I use a cortisone cream on it when it appears because it is itchy. Its like I have a virus trapped in the nerve that runs along this spinal area. If not treated with cortisone, it becomes intensely itchy and of course worse, once I scratch it. It used to be the size of a quarter. What is it? Thanks.