See the dentist. I would go back to see the dentist who performed the root canal. There may be an infection. The dentist can then best let you know how to manage this.
Not usually. Minor swelling of the gum tissues may occur from the trauma of an injection. Treatment of the root canal should not cause visible swelling. This condition may be representing an acute abscess developing. It would be best to return to the doctor that treated the root canal for an appropriate evaluation and treatment.
Maybe. You should return to the dentist to make sure that everything is ok. Until then you can use ice either inside or outside your mouth (on your face) 20 minutes on/off.