You may have some caries left. Or new one due to some reason. Or food accumulation over there due to some gap.
So should consult a dentist immidiately and get your x ray done.
X ray will help you know your exact problem.
Must do.
Next Steps
The reason may be personal hygiene or
new caries due to some reason leading to pain.
Health Tips
Do the x ray to some dentist .
Repat the root canal treatment if the dentist recommends.