Jack, Polly has blocked tear ducts. She has a mucky discharge at the inner corner of her eye. If you click on the pic in the cat gallery, you'll see the mark it leaves even after I've wiped her eyes. Are you cat's like this, or clear?
One of the vets at the practise I go to specialises in eye problems and took a look at Polly, also running some tests. She's okay, apart from the blocked tear ducts caused probably by flu in her feral existence.
Your cat may just have something irritating her eye, like grass seed/pollen, she may even have hayfever.