Sounds like bumblefoot.
For the next four days, I would
Give her an Epsom salts poultice twice a day
followed by a slathering of triple antibiotic (NOT the kind with painkiller, which is a problem for ducks)
if the swelling does not begin to go down, you would do well to get her to a vet. If that is not an option, there are threads on the Duck Forum that explain how to open a bumble. However, that in itself poses a serious risk of further infection.
If the swelling does begin to go down, continue this treatment until the bumblefoot is completely healed. This could take weeks, but it is worth the effort to save the duck.
In any case, don't delay, as this infection can go systemic and require oral or injectable antibiotics.
Epsom salts poultice:
1/4 cup very warm water in a shallow dish
2 tablespoons Epsom salts
very clean rag or a couple of large paper towels
dissolve the Es in water, fold the rag or towel so it is about 3 inches by 4 inches
get someone to hold the duck (sometimes I can do this myself, so you may be able to do it solo)
get into a comfortable position with the duck
wrap the towel or rag around the duck's foot for 3 to 5 minutes, as long as you both can stand it
don't let the duck ingest any of the Es, it is a laxative
You might be able to get away with putting the duck's foot in the shallow dish of Es water
