Has she been standing out in the cold? (Just like us, if a duck’s feet get really cold, it hurts to walk, and no, not all ducks will sit down and pull their feet into their feathers. Some of mine do, some don’t.)
Has she picked up a splinter?
Has she been stung?
Has she recently taken a big jump or tripped or gotten her foot stuck?
Has she had a conflict with another duck?
Does she have bumblefoot?
Does she have a pinched nerve that is egg related?
Did she get one of her claws caught in something?
Has she been bitten by a larger animal?
Are there drakes that have been overmating?
Is she eating, drinking, pooping?
Get her into a tub of lukewarm water where she can float - watch closely. Examine for injuries. Is the leg warmer than the other? Does it feel different?
I have read that ducks are not susceptible to Marek's ( source cited was Merck Veterinary Manual).
How old is she? Ducklings can have niacin deficiency problems. If she's older, that is not likely.
If there is no apparent problem such as a splinter or bumblefoot, I would wrap the leg in an Epsom salt compress three times a day for ten minutes, followed by a tiny bit of T-Relief cream all over the leg especially at bedtime.
