I dont know much about ducks. I suspect it could be cocci. Purchase either corid 9.6% liquid solution or corid 20% soluable powder at a feed store, it'll be in the cattle section. Dosage for the 9.6% corid liquid solution is 9.5cc's per gallon of water for 5 days. The corid 20% soluable powder dosage is 1 teaspoon per gallon of water for 5 days. Make a fresh mixture daily and dont add anything else to the mixture. You'll most likely have to administer the treated water via an eyedropper as she is most likely too sick to drink it on her own. Do this at least 5 times a day until she is able to drink the treated water on her own.
Cocci can kill quickly, give her milk or buttermilk to slow the effects until you can get corid.