She may have gotten into something poisonous.
Is she walking normally? Botulism poisoning causes them to look like they are drunk.
I would give her activated food grade charcoal in water right away. You can get it at drugstores, often, or health food stores. Or, if you burn only clean wood, you can grind up charcoal from the woodstove. Do not use the briquettes for grilling, they have nasty petroleum-based chemicals in them to make them burn faster.
There is a sticky for doing a digestive system flush, if you think she got into something.
Another thought is a sting. The charcoal may help with that, also.
I would put a couple of tablespoons of powdered charcoal into a pint of water for her.
It won't hurt her or any of the other ducks, either. It absorbs toxins.
Is she shaking her head as if to try to get something out of her mouth? Any swelling that you can see?