Sometimes my cat tenchi ends up with one eye being swollen and a bit runny.. When we had a different vet than we did now everytiem we'd go to find out what was wrong it'd be like $300. ..
Not that paying money to make sure your cat is in fine health is a bad thing, but imho Petsmart\\Banfield is a bit scammish and they do unneccesary work for something so small.
I will take a paper towel or soft cloth and put it under warm water and wring it out.. then gently wipe away the clear tears from Tenchi's eye. If anything changes- if the swelling gets worse or if there is discoloration or anything besides clear tears\\mucus coming from his eye I know immediately to go to the vet.. but usually ina few days the swelling goes down and he is fine again. We always monitor our cats very closely when something like this happens and the first or second time you tend to freak out about it.
Is it possible he bumped his head on something? Are there other cats that while playing may have accidentally scratched him? Is it possible he wanted attention at night while you were asleep and you accidentally poked him in the eye? I think i've done that when too groggy feeling for kittys head..
In any event always watch really close and if anything looks worse instead of healing better (When his eye heals it may scab over where it is pink and not grey a little that seems normal) definitely see the vet!