Usually in the course of preparing a tooth for a crown most of any
prior fillings are removed anyhow.  But it's usually a good idea to
remove all old fillings to make sure there isn't any decay, pulpal
exposures, etc. lurking beneath.
    However, sometimes when there is a deep filling on an obviously vital
tooth, I will leave part of an old filling in if I think removing it
will endanger the nerve.  Generally it won't be much filling, and I
don't think there is anything necessarily wrong with this approach, so
long as any remaining filling is not at or near the margin of the crown.
Sensitivity to chewing may indicate a minor bite discrepency which can
be relieved by a slight adjustment.  However, it may also be a sign that
root canal treatment is necessary.
    Naturally, most people will want to avoid root canal treatment if
possible.  If I think a tooth being crowned MAY need root canal, I
sometimes make the crown and cement it with a temporary cement.  That
way, if the root canal becomes necessary the crown can be removed easily
to do the root canal, rather than having to drill a hole through that
nice new crown.
    In any case, go back to the dentist to check this out.

Steve