I am not a chicken person, but a duck person. But here are some thoughts. Was she stung? Perhaps a bit of something got stuck near the corner of her eye and it is inflamed. Since ducks like to dunk their heads, usually they take care of getting stuff out of their eyes by themselves.
I might try some Vetericyn either eye gel or even spray (light mist over that side of the head). If it is a sting, you might look into whether chickens can tolerate something like Benedryl. A vet told me that a tiny tiny bit of Benedryl right on top of a sting was okay to prevent swelling.
Normally when I see swelling of the face or around an eye, I think it is probably a respiratory infection that has caused a sinus infection with swelling. Usually it is from MG or possibly coryza if there is pus in the eye and a bad smell. However, there was a case recently that seemed to be from a tumor or growth. Also insect stings or peck wounds can cause similar swelling. Watch for any additional symptoms such as foam or watery eyes, nasal drainage, coughing, sneezing, or chest rattles. Here is a link about the common respiratory diseases and symptoms:http://edis.ifas.ufl.edu/ps044
Here is the thread about a similar swelling (with pictures) that did not respond to any antibiotics as a respiratory disease should have: https://www.backyardchickens.com/t/923519/swollen-head-syndrome-is-it-contagious