Okay This only started to happen last year, but not often, and I got it again

On my shoulder's I have theses pimple-like things. They are fairly big, and are black... They where hard to get everything out, but the blood is dark red and thick (for what I remember)
It would take a few days to get rid of it all and then when it heals, it leaves a small scar. They have only been on my shoulders. I am not sure if it has something to do with the sun, but it only happened during Summer.
Please help, I am not sure what it is
Thank you