Cracked corn is not a good diet she needs a balanced poultry diet. Can you buy a poultry feed? I am thinking your not in the US?
Even rice isn't a complete feed. Cracked corn is like candy it's a nice treat. Rice is a carbohydrate your duck also need protein along with the carbs. Canned salmon...
Baytril is a good one here is the dosage 0.05ml per pound 2X a day for 5 days. Egg withdrawal is 14 days.
Amoxicillin is another
https://www.veterinarypracticenews.com/treatments-recommended-for-avian-respiratory-problems/
https://www.shagbarkbantams.com/the-many-uses-of-oxine-ah-animal-health/
Possibly a respiratory infection. It could be aspergillosis hard to say only a vet could determine that by examine. Is it possible for you to get antibiotics?
Does she have a bowl deep enough to dunk her head to clean her nares? what are you feeding her?