I can only guess, but I think you may have folliculitis (blocked pores) that are leaving dark pigmentation from inflammation, aggravated by your squeezing the bumps. I recommend that you leave the bumps alone, apply an antibiotic like bacitracin daily, and show the spots to a doctor next time you go. The marks may take 6-12 months to fade, but if you squeeze them, they never will.....
Best.
Dr. Rockoff