She may have internal parasites (gape worm, perhaps?), or a bacterial or fungal infection, or pneumonia, or some kind of virus. I don't think you can diagnose without help.
She sounds like she needs urgent care. For choking, Storey's Guide suggests
"When a duck obviously needs your assistance, pull its head forward until it is in a straight line with the neck, open the bill by squeezing with the thumb and index finger on both corners of the bill, and push your...
Did you look at that newsletter?
http://www.majesticwaterfowl.org/newslettercurrent.htm
If it is fungal, you need fungal treatment.
If it is bacterial, you need bacterial treatment.
If you cannot get a vet's help, then you'll need to make some guesses and try some things.
At times...