I wore unwashed lace underwear I ordered and I was sweating A LOT now I noticed a few bumps at the top of my butt crack on just one cheek, what is it?

Skin bumps. If these bumps are itchy they may be an allergic reaction to the material in the underwear and applying a cortisone type cream should help. Other causes of bumps include herpes simplex or zoster(shingles). If new bumps erupt or the original ones don't resolve, see your physician for diagnosis and treatment.