The past few months my back legs and butt have been breaking out in pimples, im not sensitive or allergic to anything and now theres dark scars, help!?

Folliculitis: From the description, it sounds like you may be having outbreaks of folliculitis or infection around the hair roots. Even when the infection clears, you develop post-inflammatory hyperpigmentation which are the dark spots. A common cause is too tight jeans which rub on the hairs and cause irritation. Try wearing looser or softer pants and if that doesn't help, see your dermatologist.

Laundry detergent: Did you change your laundry detergent or fabric softerner? They cause a rash as you pull up your underwear. The dark scars are the residual from the skin inflammation. See your doctor about the rash or a dermatologist should be able to help.