Swollen cheek immediately after dental procedure: Dentist could not explain why.

Went to dentist yesterday to get a permanent filling and temporary crown after root canal procedure. The procedure was in the upper right side of my face. When the dentist is almost done with my procedure my cheek begins to swell pretty bad, especially on the bottom part of my face. Which is strange since the work was done on upper part of my face. Dentist could not explain why. Woke up today with same swelling. HELP!