There are many things that it could be, the first and utmost is rhinotracheitis virus and calicivirus which appears to be an eye infection, but this eye infection is indicative of a corresponding upper respiratory infection.
With you saying that it is swollen, rhinotracheitis virus and calicivirus does not seem likely. This infection is generally spread from cat to cat,
The other is conjunctivitis but this is a discharge. I am not a veterinarian but it is more than likely allergies or your cat may have run something in it, needless to say you should take your cat to the vet and have it checked out along with the things I have listed there are many more that it could be. If you would like to research this a little further please try
Pet Education.com this is a great site when you need a correct answer for a pet question. I am not a vet, your best bet would be to see one ASAP. (O: