Glad to hear your cat is ok. The fact the vet said he was healthy is a good sign as is all the normality in his life.

When our dog was young he had one night where he was going the toilet constantly with lots of blood. He couldn't keep it in. We had to get him to emergency care. All it turned out to be was some sort of infection picked up and after some meds he was ok again. Since they spend so much time nosing around outside it can just be one of those things that happen to them through their lives just as coughs/colds/minor illness/infection tend to be for us.

My dog loves the vets though so he is very easy to get treated. While the other dogs are cowering behind their owners legs our dog is dragging us into the room because he just sees it as extra fussing! My GF's cats are nightmares so she would understand how traumatic it can be to get them there.

It may just be an infection or something minor like that.

Our dog is the same if he has had a sedative for an op. He will just sleep it off and wobble about a bit for a time and then he's back to normal again.