How old? Sounds like an injury. Get some Epsom salt. Do not let the duckling eat or drink it - it is a laxative.
Put a tablespoon of Epsom salt in a half cup of warm water. Put a clean rag in the water. Wring it out till it is wet but not dripping. Hold the duckling with the rag wrapped around the leg - not real tight, just so that the rag is in contact with the foot and the leg.
If you can find T-Relief cream, put a tiny tiny dab of that on the leg and rub it in. But the Epsom salt may take care of it.
Leg injuries can take a while to heal. In addition to the compresses, time floating in warm (not too hot) water will help.
You may need to separate the little and one of her brooder mates from the rest, as she may be getting trampled.
