That was probably due to a ruptured spleen.
A person can seem fine after a car accident and then die later.
When I was 6 years old, I was in a car wreck that killed my mother. My little sister was 4 years old, had a ruptured spleen and my dad realized something wasn’t right and insisted she be checked.
She was in a coma for 33 days and then made a full recovery.