What Does A Spreading Itchy Rash On Buttocks Indicate?

I have had a rash on my left butt cheek for 8 months, it has now started spreading a little higher and to my right cheek over the last couple months, it itches like hell sometimes, and sometimes it doesn t. The edge of the rash is like a line of tiny bumps that is very pronounced, on one side theres a rash on the other there isnt, and all through the center there are little red bumps.