If you can get her to where you can pry her eye open, look for a cloudy white spot on her eye. If you do see one, that means that something had scratched her eye. In that case you need to give her some type of antibiotic ointment that doesn't have steroids. Steroids slow down the healing when the eye has been scratched. You can usually pick something up over the counter at your vet. Twice a day, for that type thing. If you have a fly mask and she'll tolerate it, you can use that to block out some of the sun, as it irritates it.
If there is no cloudy spot then she might just have an infection, which could be treated with the antibiotic ointment, or even antibiotics if need be.
We've had several horses with this type of thing and the major thing is to keep the sun off it, and to be consistent with the ointments.
You can get a vitamin at your local grocery store called CoQ-10 , which we gave to the horses with annoying, everlasting eye issues and it really helps clear it up. You give 2 twice daily with a handful of grain, or in their normal grain ration.
Good luck. Be patient. Sometimes it takes forever to clear up.