Same thing happened to my black kitty Black. Vet said to give him allergy medicine, it worked. Came back a couple of times after that the allergy medicine worked again. His was slightly less swollen, but other than that looked exactly the same.

Thanks for all the responses.

This morning, the new swelling has subsided almost completely- much faster than the first time.

Yes, she has plastic water and food dishes- the same ones she's had for 11 years.

For the most part, she seems to leave these swollen lids alone, although she will rub at it from time to time.

I considered mites. The jury's still out on that one.

The fact that its only one eye or the other leads me to believe that it's not an allergic reaction, although it certainty could be despite the very localized reaction.

As I said, our initial impression was a bug bite, most likely a spider.

The litter get replaced, then we'll see what happens. The key thing is that other than a swollen lid, Ava exhibits no symptoms.

I was told by my vet that the exact same looking swelling was from allergic reaction. Only affected right eye. So it is possible to be allergies.

Unlikely it’s mites if it subsides and shifts locations. That tends to stick around, and while the body formulates a defense and tries to maintain, your kitty would likely be grooming that área excessively- or at least to the point that you or your wife might notice and comment on. If everything calms down that quickly, it’s an immediate insult that her immune system addresses and resolves.
Maybe send a note to slabsides. I really think he’s the member you’d get the best info from. My background is as a bird, dog, exotic cvt, haven’t done much with pet cats, just the big ones.
Hopefully it’s nbd.

From the picture and description the best I can do is a contact dermatitis. Bug, spider, or environmental. Most of the time I don't get the luxury of ever knowing. If it were allergic it would be most likely itchy.

Nothing wrong with using cortisone cream in kitties. Might clear it up a little quicker.

Tommy aka a veterinarian

Allergic reaction to a flea bite?
Hope the poor critter improves shortly.

My cat had a similar problem with an infected upper lid from her scratching and nicking the lid with her back foot nails. Antibiotics did clear it up.

Looks just like my black kitty. Get stung by something maybe?

Our indoor cat had similar symptoms. The vet said it was most likely a reaction to a house plant. The cat loved to poke around any potted house plant. Got rid of the plants and she never had the issue again.