Why am I suddenly getting pimples all over around my arms and top of my back?

there are many reasons as to why a person breaks out, stress, hormones, diet, not enough water, clothes, or detergent even, so if you can get a loufa sponge that has a handle on it, you can exfoliate and help keep the acne under control, use it on your arms as well, and use a moisturizer on your arms, it will help!