Is it in one or both eyes? Mycoplasma gallisepticum or MG, a respiratory disease can cause eye bubbles and sinus infection. MG supposedly is about everywhere in backyard flocks, and whenever there is stress, such as very cold weather or molting, they can show symptoms. I have a hen who has never sneezed, had swollen eyes, or any other signs of a respiratory illness, but she will get foam in both eyes in cold weather. Somewhere your hen may have been exposed by another chicken or wild bird.
If she gets more symptoms, such as cough or sneeze, gasping, a puffy eye, or rattly breathing, I would consider giving her an antibiotic, such as Tylan 50 injectable, but give it orally. Dosage is 1 ml for 5 pound hen, 1.5 ml for 7-8 pound hen, 2-3 times daily for 3-5 days. During and after, give her a tsp of plain yogurt daily for probiotics, and mix it into some food with a little water.