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 edema, and ocular discharge, among other respiratory symptoms. Luckily, MG isn't that common in waterfowl as much as it in chickens. More likley, she either got stung, or got foreign matter in her eye, or got poked somewhere on her skin, and the whole area is infected.
I will wait for you to answer the questions, as treatment will vary with the physical examination findings.
