I Have A Read Almost Purple Looking Rash Just At

I have a read almost purple looking rash just at the top of the buttock crack area, literally right where the crack starts. it goes up a little to the bottom of my spine and spreads on the outside of the buttock cheeks at very top. One Dr said it was some and prescribed medication and primary care DR said it was just E coli. None of the meds have worked. It gets kinda crusty and dry but stays