Welcome to the ehealthforum and I am glad to help you out. Causes of such a buttocks rash include moisture(due to sweating or frequent washing), the abrasion caused by clothing,the pressure of sitting,chemical irritants like laundry soaps, colognes and scented toilet paper,certain food irritants and skin problems such as psoriasis, seborrhea and eczema. I suggest you to get an evaluation done from a dermatologist to confirm the diagnosis as without examination a diagnosis cannot be made.
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