You need to talk to your treating doctor to find out the exact reason for this.
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it could be due to multiple reasons..
the doctor may have missed an extra root canal in the tooth which was left uncleaned..
over instrumentation of root canals which must have irritated the root tip area causing pain
improperly done or incomplete instrumentation of the root canals..
sometimes the medication or dressing may not reach the tip of your root due to improper cleaning of the canal system..hence it is essential that the doctor cleans all the canals following all the protocols
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practo consult me for furthur discussion