First secure the cat so you can examine the cat's eye. If you find an injury to the eye you should probably just call your vet and take the cat to the vet right away. If you don't see any injury you might try flushing the cat's eye with clean water.
Take the cat to the vet, there's obviously a significant problem with his eye. Yeah maybe your dog has scratched it, or maybe he's picked up an infection.
my suggestion for this problem, without being able to see the cat, is to take it to the vet. There are too many reasons that this could happen to guess.
