My cat had a couple days where her eyes watered. It looked like she was crying; they were literally tears, not discharge or sneezing or anything. I was worried, took her to the vet, and they told me that she most likely had an allergy or had gotten some kind of irritant in her eyes.
If there are no other symptoms, then that may be what's going on with your cat. Though mine cleared up quickly, whereas yours sounds as if it's been happening longer, right?