Third eyelid showing in one eye only

So I'm aware that third eyelids showing can be a sign of worms, infections, or pretty much anything being wrong with the cat. But when it's only showing in one eye (about 1/3 - 1/2 of the eye) does it still mean illness, or is it more likely to be an injury in that one eye?

It could be the start of an eye infection in just the one eye. Or for whatever reason the cats body only reacts with one third eyelid to things because of whatever reason.

Either way a visit to the vet is probably due to see what is going on and take the appropriate actions.