This could have a lot of causes. You might be describing a corneal abrasion or corneal ulcer. You might be describing an allergy related issue. The list really is endless.
The only thing that you can do for this at home is flush the eye with saline. Then blot it dry very gently with a clean cloth.
Then see your vet on Monday for a good ocular exam. Eye health is very important at this age.
Let me know if you still have questions.