What happened to my rear? Painful firm zits on it.

Butt acne. There are several factors which results butt acne: use of bad soaps or detergents can cause acne certain perfumes and scents can cause allergy which may run up to the butt, excessive sweating due to tight fitting clothes treatment: minocycline, doxycycline, application of alcohol in the area and then apply Benzagel in the affected area with a tissue. Then rinse it of with water.
Folliculitis. Not due to perfume or soap but may be related to sweating or tight clothing or prolonged sitting, especially on plastic. OTC benzoyl peroxide gel may help. Careful: it bleaches colors out of fabrics. Antiperspirant wipes (Certain-Dri) may help. You may need to see a dermatologist.