It could be a cracked tooth, that would mean extraction probably and an implant to solve the problem. Or it could be a missed root canal and need re-treatment or its just not healing and extraction best.
Another reason for a root canal treated tooth to be painful is, if the 'bite' is too high. This means that the tooth is taking alot of biting force and the ligament of the tooth gets painful (like a sprain).
1) infection is still present
2) missed canal
3) fractured tooth
Would have to have an endodontist evaluate it.
1. The best way is to go back to your dentist to check. He or she may know your history and get answer right away.
2. It may be re-infection, crack, or missing canal...etc. Sometimes it may need to be extracted.