Just found this - I would get her into a tub of lukewarm water deep enough to float in and watch carefully. She may have just pulled a muscle. Painful, slow to heal, but not too serious.
If there is a drake, they can really hurt a duck by mating roughly. You may want to separate them.
An Epsom salt compress (don't let her drink the liquid - it is a laxative) wrapped around her leg for as long as you can stand it, followed by a little bit of Traumeel ointment or T-Relief ointment on the leg, twice a day - at least once a day - and if it is a pull , ought to eventually heal itself.
Worse might be egg binding - lukewarm tub of water, but gently feel around vent and belly for a stuck egg. Give about 100 or so mg of calcium (calcium citrate pills dissolved in water, calcium gluconate liquid, or even half a Tums dissolved), a few times a day is what I would do - I am not a vet.
