Anything is possible.  But usually cold sores are reactivation of a
virus that has been acquired years before.  During periods of stress
(illness, dental treatment) the virus can emerge along the nerve and
cause eruptions on the skin.  Dental treatment has been seen to do this.
    Of course, if there's a possibility it is something else, it could be
accidental trauma to the tissues while working, or even accidental
biting of the lip while it was still numb from the anesthesia.

Steve

The worse thing my dentist ever did was cause my wallet to be empty.

only if you kissed your dentist.
The difference between herpies and true love is: herpies lasts forever.