Little bumps that pop out on your buttocks or butt cheeks can be cause by tight fitting clothing. Tight fitting clothing can slow the skin's natural exfoliating process and lead to the formation of bumps. Normally, bacteria sit on the surface of the skin, but when you wear tight fitting clothing, it can rub the bacteria into the pores and cause breakouts. Also any sweat that you may have from sitting provides a moist breeding ground for the bacteria to grow.
To remedy this, wear looser-fitting clothing and wash the area with a mild soap. You should also avoid using fabric softeners in the dryer because they leave fibers on your underwear which can irritate your skin further.