There is a condition in which oil, like the kind you work with, causes an acne-like breakout. That's possible, although I couldn't say why it should start breaking out just now. I can't see any clear connection with the cortisone shot.
I don't think it's anything to worry about, but I do think you should call back the dermatologist who saw you for some advice. Perhaps an oral or external antibiotic might help, or maybe some changes in what you wear when you work.
Take care.
Dr. Rockoff
Dr. Rockoff