I would look at the bottoms of the feet for signs of a splinter or lump (they have little callouses, those are normal).
Otherwise I suspect a pulled muscle.
For that, Epsom salt compress followed with something like T-Relief or comfrey salve rubbed onto the leg, and pulls or sprains can take several days or longer to get all better.
Epsom salt is a laxative, so you need to use a compress or, like some folks, hold the duck while she stands in a shallow flat-bottomed bowl or small bucket. Just so she does not ingest any.