May be failed RCT , may be adjacent tooth , may be high points , reinfected tooth or simple case of incomplete cleaning of tooth in the first place
I strongly recommend u to visit a root canal specialist (an Endodontist) , if u intend to save tooth and use it painlessly..
Next Steps
Find an Endodontist(worth every penny u will spend)
Considering it is a failed or misdiagnosed case ...
Health Tips
Don’t delay .. online consulting won’t help u much