Why are my teeth sensitive to chewing?

There are two primary reasons.

Bruising of the ligament that surrounds a tooth by clenching or grinding.

Remedy:  Evaluate the way your teeth fit together with a computerized scan (T-Scan) to look for teeth which are bruised by clenching or grinding.  A minor adjustment of the bite may solve the problem or a bite guard may be indicated.

A crack in the tooth which is irritating the nerve.

Remedy: Determine the depth and location of the crack by removing the old filling.  If the crack is close to the nerve a crown is usually indicated to stabilize the crack and prevent it from getting worse.