Squinting in one eye only

My cat keeps squinting in one eye only.  It's like he is winking, and it's not all the time but sometimes. And sometimes that eye will look smaller than the other eye..  This started three weeks ago on December 10.  I took him to the vet on December 12 and they said everything was fine and they could not see anything.

On December 3 he fell 12ft from an upstairs landing and landed on his feet but his nose was bleeding a little bit.  The vet had checked him out on December 4 and said he appeared to be fine.  The vet said she though he had bonked his face and that is what caused his nose to bleed.

I don't know if the two events are related.  The eye squinting didn't start until 7 days later.

What could be wrong with his eye?   Thanks.

He could have conjunctivitis, an eye infection or it could be allergies or it could be a scratch.  I would ask the vet to do a stain to rule out a scratch and then ask for some eye drops in case it's an infection. 

Thanks. What is a stain?

It's a test the vet does to check for scratches on the eye. 

http://www.petplace.com/article/cat...ts/tests-procedures/fluorescein-stain-in-cats