Don't panic just yet

If she is limping then the most likely possibility is that she has just hurt her leg through general duck clumsiness. Leg injuries are very common in ducks - they seem to go out of their way to trip over and run into things. An injury from a cut or scrape is possible too.
If you haven't already, catch her (move slowly to avoid making her run) and settle her down then have a good look at both her legs and feet. You are checking for anything that looks out of alignment, is unusually hot compared to the other leg or is swollen. Also check for any cuts or any point at which something may have punctured her leg or foot. Check to see that all her toenails are there - sometimes they can get one caught and pull it off and that will cause limping as well. If a toenail has been pulled off you will possibly just see the 'quick' which will look like a little pointy jelly thing.Don't touch it because it will hurt.
Also look under her feet for any signs of large, swollen lumps with what looks like a scab in the middle - that can indicate an infection in her foot and that kind of thing can cause limping. If you see that then do a search for 'bumblefoot'. All ducks (except babies) will have callouses on their feet from walking around, which is normal (especially on the heels). Rough callouses are nothing to worry about - what you are looking for are large, swollen looking, rounded lumps.
If it seems to be a puncture or cut that looks swollen and infected then she will need antibiotics from a vet. If it is a toenail that has been pulled off then don't worry, it will heal by itself. Just try to keep her on clean litter and keep swimming water as clean as you reasonably can. If you can't see any visible signs of an injury or swelling anywhere then it's quite likely that she has just strained it. In most cases strains and sprains will heal up - if it is that then you should notice improvement over the next week. Just try to keep her quiet and away from drakes that might chase her. Swimming is good exercise for strains.
Let us know what you find when you examine her closely
