I'm so sorry to hear about her injury, but also so glad she is alive.
Mammal bites are filled with bacteria that is poisonous to birds. Even after the initial healing a dangerous infection can continue. Internal damage such as injured organs or broken bones can also be an issue, as can lowered immune system responses.
We had two chicks that were terribly injured by a hen. They made a full recovery, then caught a virus and died from paralysis. It is very sad, but sometimes an injury can cause more damage then first noticed.
To help your duck I would suggest getting her somewhere quiet, neither hot nor cold, and getting her drinking electrolyte/vitamin water and nutrient rich food to help her body heal. Let her rest, check her for wounds and clean any you see. You could even try antibiotics or some other medication to help her fight off any internal infection. Whatever it is, it may just have to run its course. All you can do is support her during this to help her fight through it.
I hope she gets better soon. Best of luck to you and your family!
