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.

Related Questions

Have bad canker sore from recent cavity filled tooth irritation of cheek. Am not feeling well and have fever of 100.3. no tooth pain. Is this normal?

No. but then you may also have a cold which may account for your symptoms. If the problem worsens or persists for more than 10 days, be sure to check with your doctor. Read more...
Canker sore? Canker sores do not cause fevers. You may have a viral infection...cold or flu. If the canker sore does not heal in 7-10 days, have it examined by your dentist. If the fever persists, see your medical doctor. Read more...