What Causes Sudden Onset Of Painful Rashes On Buttocks And Thigh?

I occasionally get a very sudden painful red rash that seems to look like sores either on my butt cheek or outer thigh. It always shows up suddenly. Literally even just an hour prior to it, there is nothing and no symptoms...not even redness. The first time I thought I must have somehow gotten a brush burn, then I realized that it looked like an irritated cluster of popped and scabbed over blisters. I never had any diseases that I knew of until roughly 4 years ago when my annual exam revealed that I carried the herpes virus that the doctor said would show up as sores on my lips. I was shocked and the doctor asked me if I ever had sores on my lips. I never have...even still. She was surprised and I was offended. So anyhow, I popped up with this sudden painful stinging blistery rash today for the second time this year and decided to take a pic this time. Looked online to compare it...and it looks like one of three things. Impetigo, herpes or shingles (which I guess is still herpes and yes I had chicken pox as a child .) I seem to get this rash randomly about 4 times per year. Although it seems to pop up in just minutes...It usually goes away in 7 days on it s own. IT does seem to cover more area each time it does show up and that scares me. It never leaves a scar. I just keep it clean and put hydro cortisone on it to get rid of the pain...but I can honestly say that I don t know what it is. I would like to know. I did see my primary about it once...but by the time he could give me an appointment, the rash was already gone. He couldn t diagnose it. I wish I could submit my pic. Hopefully I will get an answer that helps.