Oy my left ear piercing hurts. Keep stud in or take out?

Depends. If it is sore from a recent piercing but is not getting worse you may be able to just watch it. However, it is more likely infected and should be removed. I would recommend you see your doctor as ear-related infections can be due to pseudomonas bacteria.
Take it out. If it continues to hurt, you might have an infection, and you should see a doctor.