If you’re having any pain or swelling from a tooth that has had root-canal treatment, contact your dentist. Sometimes after a root canal a pimple develops near the tooth. This pimple will often go away and then come back. This is called a fistula. You may notice pus draining from the fistula. The fistula is a sign that there is an infection and your body is draining it out through the pimple. There is usually no pain in this situation, but you may notice a bad taste or odor in your mouth.