Should I go see a doctor? Any guesses on what's wrong?

This would not be normal unless your muscles were so over-taxed from playing that they just gave out. I would guess you have caused an injury in there and your family doctor probably needs to evaluate this and see if you need a referral to an orthopedist. You can wait and see how it does overnight but if it is swollen and painful tomorrow, you need to start taking some motrin regularly and see your doctor.