Some ducks are more sensitive to a lack of nutrients, so that may be contributing.
She may have a systemic infection, also, or a large parasite load.
You may be able to purchase a poultry vitamins and electrolyte mix, since even those who raise poultry and waterfowl for meat may create a demand for something like that at a feed store.
If you cannot get poultry vitamins, there is a product called poly-vi-sol which may be sold where you are. It is a multivitamin. You would need to calculate the proper concentration of vitamins (since ducks are so tiny), or you may be able to search the archives here for dosages.
If she has an infection, antibiotics may be needed. I have used Tylan (tylosin phosphate), which is dissolved in their drinking water. Eggs are not to be eaten during treatment or for ten days after, to be on the safe side.
For parasites, some use a natural method, combining diatomaceous earth (food grade) and garlic, but be careful as garlic - according to some sources - can be toxic to ducks. I feel that it may be dependent upon how much is ingested, but that is just my feeling, you need to sort this out. There are also de-wormers such as Ivermectin (stromectol), and again you may need to calculate a dosage that would be appropriate for a little duck.
Keep her warm, well-fed (better than well-fed, she needs high quality food right now), with loads of fresh water as you sort out the problem.
If she has peritonitis, it could be fatal, but please don't give up on her when there are so many options to try to help her.
