How typical is it to get a canker sore after a cavity filling?

It's not normal...: But, it's not uncommon either. Dental procedures can be stressful. That stress is what causes canker sores. Limiting any other physical and psychological stressors while receiving dental treatment may help reduce the episodes of canker reoccurrence.

Possible: Trauma to the mouth may trigger canker sores. There is some speculation that the same germ which leads to dental cavities is also responsible for the canker sores.