I have a duck that went thru that. I read up on it and with ducks at least, a small cut in the skin of the foot can let in bacteria. In some instances the bacteria takes up residency in the joints of the leg - causing inflammation and pain. Talk to a vet. She sounds like she is in an acute phase. With my duck, he got over the infection but had residual damage. He's still with me and when it gets cold, he limps. Other than that he's fine. The rapid onset of symptoms tells me infection - not arthritis. I may be wrong, but I'd get her some more help if you can swing it.