hello, with your kind of history, the most likely diagnosis looks to be keratosis pilaris which usually starts appearing in puberty as rough, slightly red, bumps on the skin often appearing on the back and outer sides of the arm and can also occur on the thighs, hands, and tops of legs, sides, buttocks. Although, having pimples ( acne vulgaris) is rare on these areas but it can be a remote possibility. Also, rule out any ingrown hairs or infection on hair follicle (folliculitis) due to shaving or waxing hairs over these areas In such kind of patients, I usually recommend tretinoin cream in night and if there are pus containing lesion, then apply an antibiotic gel like clindamycin/ nadifloxacin/ erythromycin. Thanks Dr Jyoti Gupta