Pain in 1 canal during RCT - Thoughts from the pros?

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During a root canal on #18, I experienced sharp pain during drilling/filing in 1 of the canals.

The other 2 canals were pain free / numb.

The pain was very SHARP, after the file was removed, a dull pain was left in my jaw for several minutes.

The dentist stopped several times to numb me more and more and more.

I took 7 days of antibiotics before the treatment as well.

What do you think caused this? Thoughts?

 

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Update #1 - Part of the reason I ask is a curiosity of why the pain existed. The other reason I ask is that there is still some sensitivity in that tooth, 4 months after the treatment.

Chewing causes no pain. Pushing causes some pressure, but no pain. But TAPPING on part of the tooth causes some sensitivity.