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These few particular pimples(?) on my shoulder have been there for about two years now. I've attacked it with all the treatment I normally use (tretinoin cream and benzoyl peroxide) and it doesn't seem to be affected at all. They don't hurt like normal pimples so I don't know what's up.
I don't think they're scars because they're an actual bump, not just discolored skin. 
Hi, so I've repeatedely visited this Thread hoping for an answer to this since I have the same exact problem!
I've spent months Googling different key words and finally stumbled on an answer! Not sure if you're still searching for one, but for the others who are still desperately trying to identify it...
It's not acne, but a mildshoulder keloid.
Several characteristics of keloid are raised, shiny, painless, discoloured (pink to red to light reddish brown) and irregularly shaped, that never subsides in size for months or years (mine has been on my shoulder for 5 years now!) It's usually caused by inflammation such as acne, and can spread out around the area.
As for treatment, it varies from surgery to laser to injections to ointments. But I read that surgery could bring it back since the wound is susceptible to bacteria or something like that. I've also heard that injection is the best, but I'm not sure nor am I a doctor.
Hope this helps!