I had a scare years ago where my kitty wasn't able to go to the bathroom and was becoming lethargic and she looked miserable. I took her to the veterinarian and I was shocked by how few options they had. They were very experienced and genuine people, but it was clear they had a very limited set of options and sadly most roads don't lead to great places. What the vet did determine by poking a small plunger thing up her butt (no joke) is that what very likely happened is that she jumped from something and fell onto something which caused temporary swelling. The plunger thing pulled some discharge from her rectum that shouldn't be there and is basically what causes the swelling. Thankfully she was back to normal in a day and I've never seen her that miserable.
(For anyone wondering, she is an outdoor kitty with fields and trees to climb and mice to hunt)
Obligatory cat photos of her now:
https://i.imgur.com/W1M2hzW.jpg