I am sorry for your loss. Considering when it happened you were just going back from the vet, if even someone who studied and work all day with birds did not find anything, it is normal you didnt find what was wrong either.
Anyway what I am saying is not going to solve anything, except that you do not have to blame yourself, you did your best.
Anyway what I am saying is not going to solve anything, except that you do not have to blame yourself, you did your best.