Have you added any new chickens to your flock recently?
Have you looked in the eye - do you see any worms moving?
Does she have any other symptoms - coughing, sneezing, etc.?
A swollen eye with discharge can be caused by a number of things - a peck or injury to the eye, getting dirt/debris in the eye, respiratory disease and being in poorly ventilated housing are some of the most common.
General care of a swollen eye is to flush with saline, inspect the eye for debris, pus and signs of injury - remove discharge/debris from the eye and apply an ophthalmic ointment in the eye like Terramycin or Vetericyn eye gel.
Eye worms are normally found in tropical climates, but it's possible for chickens to have them in cooler climates. You should be able to see them moving in the eye. If you do, they need to be cleaned out and treated. Some people use VetRx, but consulting your vet for assistance and medication is always best.