Rash on buttocks but doesn't itch?

Difficult to tell. Skin issues are very difficult to diagnosis without actually seeing and touching the lesion. Occasionally, doctors may even need to biopsy or culture the lesion to make a determination. Best to see you family doctor or dermatologist to get you the best answer.
Rash. A non itchy (non-pruritic) rash may be a heat rash, viral rash, flushing. Rashes are difficult to diagnose confidently without being seen, so if you are uncomfortable, let your octor check it out.