Thank you for your insight, you must know a whole lot more than I do after having ducks for 14 years and I'm grateful you responded.
They only examined her by sight and feel, didn't run any tests like X-rays or testing stool samples.
They have her on 1ml TMS antibiotic ever 12 hours and 1ml nsaid for pain given the septic arthritis theory. I'm a little dubious though without the other tests and the fact she seemed to have a sprain rather than any cuts that could introduce bacteria.
Another thing is her breath smells a little weird, so I wondered about sour crop maybe? But I'm not sure if that would make her seem like she feels so unwell.
Fwiw, the vet said he wasn't too concerned she's an emergency case since she seems otherwise healthy and is still eating and drinking fine, but he wasn't a specialist. I'll be taking her to the avian vet first thing Monday morning, but I don't think I'll get much sleep til then. She's my baby and it hurts so much to see her feeling ill.