on bottom, upper back and shoulders!! Seems to be because of Truncal Acne
. You have not mentioned your age which might have been a clue and duration since they are occuring. They might be leaving behind postinflammatory hyperpigmentation
which is very common.
Truncal acne is difficult to cure and you need to use a soap/wash containing Benzoyl peroxide
. You can try benzoyl peroxide cream/gel which is available over the counter. Pigmentation should resolve on its own over time.