Small pimples on back side of body?

Folliculitis: What you are describing sounds a lot like folliculitis (hair follicle infection - mild). Often, the best approach is to use an acne-type body wash with benzoyl peroxide (bpo). Bpo is over-the-counter and well tolerated. Use it in the shower daily and try to leave it on for a couple minutes. If this doesn't help after a week, then see your doctor for help and guidance.