Hello,
It may happen that some residual bacteria is present there. It happens sometimes that some of the bacteria remains that causes formation of abscess(pus) in the area below tooth roots that causes pain. Or alternatively pain occur due to overfilling of the canal or any filling that is causing impingement on gums.
Next Steps
Need to visit a dentist who is treating you
Health Tips
Take a tab ketorol dt disperse in half glass of water and have it. You will get relief for the time being