A very similar thing happened to my Samson. We have a very large cat and Samson is under 4 lbs. The size difference has not stopped Samson from basically torturing Tigger, our cat, and Tigger usually puts up with it or jumps up high where Samson can't reach him (not that he doesn't try). About a month ago I guess Tigger was having a bad day and he swatted Samson, apparently scratching his left eye. Same symptoms as you described, but his eye appeared swollen shut. I immediately got on the internet, unsure if his eye condition was due to "combat". I scared myself sick reading about several disorders and diseases, so I took him to the vet that day. She put a green dye in his eye and it showed what looked like an ulcer. She said that it was due to a scratch. When she said this I felt relieved because there was absolutely no underlying condition. She gave me an ointment that I had to apply to his eyeball 3 x's a day and his eye healed in a couple of days. He is absolutely fine today and still attempts to attack Tigger daily. I'm sure you will want a definite diagnosis from your vet, but I hope this helps you because I know I was panic-stricken! The ointment is called neo-poly-bac and it worked very well for Samson, but I suppose it depends on how deep the scratch is.
Good luck and please let us know how it goes!