This sounds like a typical leg injury. Take a picture of the tops and bottom of feet to rule out bumblefoot.
Leg injuries from mating or just other accidental causes will cause inflammation at the injury and usually is hot. This does not mean it is infected, but it is a possibility. I treat mine with ibuprofen to help with the inflammation and pain. I can give you dose for it if you decide to use it. I would start weighing your duck and keeping an eye on how her keel feels. If she drops too much weight, she will need to be tube feed.
Here is how to judge by the keel :
http://www.poultrydvm.com/topics/poultry-body-condition-score-charts
Ducks with leg injuries need rest and water therapy in deep water. I would recommend water therapy a couple times a day. It doesn't need to be long. The deep water will help take weight off the injury and give some relief and allow the injury to heal.
Other things that could be going on are septic arthritis or slipped tendon.