Oil glands in the skin become overactive. Stress can trigger acne, while acne can trigger stress, its a very harsh cycle. When in stress you produce hormones, including cortisol which would stimulate your oil glands to produce testosterone.
Body acne is caused by the same part that produced facial acne. Dead skin cells are trapped within the follicle and then create the blockage.
We get pimples becuase of these glands that make sebum, it is an oil that covers your hair and skin. If there is too much sebum and dead skin cells then bacteria may get trapped inside your pores and multiply.