Is she exhibiting any other odd symptoms? Hunched up? Lethargic? Wheezing, coughing, nasal discharge, or a foul odor coming from her?
I would quarantine that chicken immediately. Monitor her breathing and intake of food and water. If she is off eating and drinking, you may have to tube feed.
It's possible she got pecked. It's possible she itched her eye due to mites and cut herself, which allowed a little eye infection to set it. We will hope that's all it is for now.
An eye infection can be treated using Tylan 50 injectable (available at Tractor Supply) - you'll need syringes and needles anyway to get the medicine out. Pull a little Tylan into the syringe and mix with saline solution (salt and warm water). Wrap her in a towel and drip the diluted solution directly into her eye. It will run everywhere, and she may shake her head while making angry noises at you. It's okay. I promise. <3
Grab a little triple antibiotic ***without painkiller* and (using your ring finger, because it's smoother), dab a little ointment around her eye. If it gets IN her eye a bit, that's okay, too. No harm will come to her.
Keep her confined while you treat her. You should see improvement in a few days.
Please let us know if any other symptoms show up.
MrsB