My skin condition?

Hey everyone, I'm new here.
I have a question about skin condition. Since my early teens, I have a "breakout" on my upper arms and back- red pimple-like condition. But strangely it lasts for years and leave somekind of reddish marks on my skin, and strangely still the condition only happen on my arms and back.
I don't have any pimples on face or chest area- even as teen if I got pimples on face they leave my skin smoothly, with no trace whatsoever. So why do these arms and back pimples left rash-like marks that wouldn't disappear for years?
Recently I got somekind of chemical burn on my neck due to wearing a shirt that's not washed properly-apparently my mom used too much detergent and didn't use enough water to clean the detergent off(she handwashed my shirt) and as result now there are some similar reddish spot on my neckline. So is it possible I got chemical burn pimples too? But why it didn't disappear after years of washing my own clothes(and I use different soaps, and I always wash with lots lots of water)?
I want to upload the pic but I'm in small town in isolated country and internet access can only happen through smartphone- Maybe I can upload the pic through mail if someone genuinely want to identify my marks? Thanks.