How old is your cat?
Is the eye red or watering?
Does he seem sensitive to light?
How long has it been this way?
This is most likely an injury to the eye. Usually a scratch, abrasion, or an ulcer. Since she was outside she may have run into some brush and scratched her eye. I recommend taking her into the vet tomorrow to have the eye stained to see if there is any abrasions or ulcers. If any are present they can send home a pain reliever and antibiotic ointment. She will start to feel better within a day or two once the meds are started.