First take a good look with a mirror all around your body from the waist down, to rule out any fungal infections. If you see any odd patches of skin (shiny, moist, itchy, flaky, raised) then you'd need to see a doctor for treatment.

If you look and see nothing abnormal, then this more than likely is the natural scent of the apocrine gland/bacteria combo, of your butt (and/or scalp). Buy a medicated shampoo such as Nizoral or Selsun Blue, and wash your hair AND body with it, instead of soap. You should notice a remarkable improvement in 2 days.