I have just had amazing results with an eye infection using unpasturized honey. Initially I used eye drops that I got from a vet but the infection returned. I have recently become a beekeeper and after attending some meetings with other beekeepers I have learned of the healing properties of honey. Not store bought honey. (most is from China) but local unpasturized liquid honey.
I know it seems wrong to put honey in an eye but my girl didn't mind and it cleared up. It took a few days of me doing it twice a day. Don't mix with water. The honey will be best on it's own. It was used a lot for infections. There is a vet in our town that used honey on wounds. I saw my neighbour treat her dog with it when it had a gash in it's side. I was so skeptical but now that I have seen it I am a believer.
But another thing that it might be is an eye worm. I had a hen with that too once. If you look real close you can see it. It is small and the eye swells but is not infected. I used Piperzine a wormer in powder form. I gave it to a big hen every day for a few days and again in a week if it is an eye worm it can have eggs and cycle around and have more worms.
Because she wouldn't drink the water when I put it in the piperzine in the water I soaked a piece of bread in the wormer powder in a little water. I kept her close to the others but in her own cage.
Good luck