Brief Answer:
Failed/incomplete root canal treatment, gum infection, X-rays

Detailed Answer:
Thanks for your query. I have gone through your query.
As far as the pain in the root canal treated tooth is concerned, I have the following reasons.
The pain can be because of the failed or incomplete root canal treatment.
The other possible cause can be a gum infection secondary to the deposits around the tooth.
If i am your treating doctor, i would suggest you to get a scan (X-Ray/radiograph) done like Radiovisiography or Cone beam computed tomography to confirm the diagnosis.
Once the diagnosis is confirmed, we can manage it accordingly by doing a re root canal treatment if there is a failed or incomplete root canal treatment.
If there is gum infection, then it has to be treated with scaling and root planing.
These are the possible causes, diagnosis and treatment plan based on the data provided by you about your problem.
I hope i have answered your query. If you have any queries, you can reply me back.
Get the scan done and upload the images, so that i can give you the exact diagnosis and treatment plan.
Take care.
With Regards.

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