You should get her to the vet today or tomorrow at the latest.
She most likely has an eye infection such as keratoconjunctivitis (KSC) or conjunctivitis. These are common eye problems in cats and are treatable with antibiotics and eye drops or ointments .
You can irrigate the eye at home with plain saline. Do not use human eye drops like visine. If it's one eye and not the other she may just have something stuck in it like grass, dirt, or dust. So try cleaning it out with the saline. If it doesn't look better in a few hours then she will vet assistance. Eye infections that go untreated can lead to permanent vision loss. So don't think it will go away by itself.
Let me know if you need further help with this.