What causes major facial swelling after a root canal?

Depends on the level: Of presenting infection or a secondary infection. Let your dentist know so appropriate measures / antibiotics / i ;d ... Can be taken. Typically this doesnt happen. Hope this helps.

Infection: This sounds like a serious dental infection that requires immediate dental care. Call and see the dentist who is treating you right away. If he\she is not available and has no covering dentist, see a physician ( 24/7 medical facility doctor or er of hospital) as serious infections can be life threatening.