Acne like pimples on shoulders and back start red and after a while turn into brown spots that stays for ever... what could it really be ?

Acne: Especially at your age, acne is likely to resolve with some postinflammatory pigment that can stay around for months. You may try topical benzoyl peroxide until you're able to get seen by your physician. You'll probably need some systemic treatment if the acne is on your trunk. Remember that an antiobiotic should never be the sole treatment for acne.