Brown Patch On Lower Back, Less Sensitive Area, Itches Sometimes. Causes And Treatment?

I have had an area around my low back and butt crack area that is a shade of brown, grey almost, for the better part of a decade. the skin feels thicker and has seemingly less sensation when compared to other parts of my body. Sometimes it itches, but not often. It does not extend into my crack or anal area...it is limited to the upper part of the crack and low back. it is about 4 inches in diameter and sometimes has bumps that resemble acne . I m 6 6 and 330 lbs. Im 32 years old. not diabetic, hba 1 c at it s highest was 5.7, but typically stays around 5.0...not sure if that s relevant