Had severe tooth abscess 3 months ago, was treated w/ root canal and was fine and now that tooth is painful again. did the abscess return?

Maybe: You can get the answer by going back to the dentist that did the root canal. They are over 90% effective... But sometime further treatment may need to be done... The root may need re-treatment... Maybe an antibiotic would work or a retro fill will work.. Talk with your dentist who can see an x-ray a determine the best course of action..

Not impossible: Should get checked by your dentist, and between exam and x-ray, should be able to determine if reaccumulation / recurrence has happened.

Possible: A root canal removed bacteria that is causing the sensitivity or pain in your jaw and tooth. Dealing with bacteria can be a difficult chore for anyone so it is possible that a root canal went bad and another infection or it is also possible you have a crack in your tooth or if you did not receive definitive treatment such as a crown, you have a leak causing an infection. Get evaluated.