Might silver sulfadiazine help?
We had a very similar problem, but our duck's entire face was swollen three times its normal size and one of her eyes was swollen shut, with a trace of blood under the eyelid. We never figured out for sure what exactly caused it, but we guessed something had irritated or injured her eye and she maybe had an allergic reaction to it. We used silver sulfadiazine and put a little dab of the paste on her eye twice a day until she was able to open the eye again. (We used something else for the swelling.)
Storey's Guide to Raising Ducks says that a foamy eye is usually the result of an injury, but can also be a sign of respiratory disease. You might want to check if she's breathing normally.
I wouldn't think it's an infection, because that usually takes longer to develop.
I believe you can use normal eye drops, but you might want to wait for affirmation from the experts.
I can't get Terramycin at all where I live, but it's just an antibiotic (oxytetracycline). If you can't find it, I'm sure some other antibiotic (such as the silver) would do just as well.