Pekins have more tendency towards leg issues because they are designed to be meat birds and often"harvested" at a young age before leg issues show up. Just normal running around can be too much for some Pekins, especially if she is a little overweight so be sure she isn't being chased by the other ducks or herded too quickly.
When any of my ducks show signs of lameness I put them in a basin of water, making sure they can either get themselves out when they want to or supervising them often so they don't get waterlogged. Getting the weight off their legs gives them is often enough to give them a chance to heal.
Try to arrange housing for your duck so she doesn't need to move around much until she is back to normal. I throw up a small temporary wire pen with food and water and lots of straw, plus the water basin for floating, usually near to the other ducks so the lame duck doesn't feel the anxiety of being separated from the flock. I have sometimes arranged for one other duck to stay with an "invalid" until she is ready to return to the flock if housing her with all the others isn't convienient.
What are you feeding your ducks? Too much corn tends to create problems with leg injuries and other issues because it is fattening and has no real nutrition. In general ducks need a bit higher niacin level than what's in chicken feed and adding some Brewers yeast sprinkled on their food or a niacin tablet dissolved in water can often help with leg weakness. Kelp also is useful for additional minerals. Ducks that have time to forage in addition to a regular poultry feed often do better nutritionally.
Hope she gets better soon.
