How does the other side look? We're you able to look inside her mouth, did you see anything? How does the swelling feel, hard, firm, watery? Is she sneezing at all, sounding raspy?
The problem could be related to a respiratory disease such as Mycoplasma Gallispectum, which can cause facial...
If her face was swelled up, it could have been a sting bite. Hard to know without pictures of the duck. If you get some when she is laying, or in the coop, that would be much easier.