I Have Petechiae On My Left Arm. I First Noticed

I have petechiae on my left arm. I first noticed it a few weeks after my son's birth two years ago. It has since stayed the same, neither getting worse or better and there are about 100 small red dots/lines from my left outer shoulder to my elbow. Just wondering what causes this. It doesn't hurt, my arm gets numb on occasion but nothing I really notice. Never seen it anywhere else. Should I be concerned? Thanks.