It may be in the back, and that could be why she was plucking her own feathers.. She started the job, and the others joined in. So something in her back is hurting. Sciatica maybe? Does it happen to ducks?
Here's just a thought that came to me. If she's able to swim in the pond, but not effectively walk, she's got some kind of nerve damage that makes it painful when there's weight on the leg, yet not in the water.