What Causes Red Rash Between Buttocks?

My mother, aged 83, has a round, red rash between her buttocks that she says is extremely painful. It does not itch. She has had it for about 3 months. The dermatologist said he didn t know what it was. He prescribed Nortriptyline, which she does not want to take due to her kidney disease and diabetes. She wants to know what it is, and how to get rid of it.