My sister's Pippa had one very watery eye on Friday, and some discharge. Vet visit on Saturday said conjunctivitis.
Not sure if cats have the same feeling as we do when we get it, but if they do I am sure that a vet visit and ointment may alieviate any discomfort.
I can't say for sure, Lexy has never experienced this so I can't be sure that is what it is, but may be worth a vet visit.
In the meantime use some cotton wool, dipped in boiled water (cooled) and gently wipe the eyes, making sure you use clean cotton wool with each swipe so you don't contaminate the other eye.
Also, wash your own hands thoroughly after.
I hope its something relatively simple, but if you aren't sure get the vet to take a look.