Hello,
Yikes. What a roller coaster.
Ok. So probably no one is going to like my advice, but here it is.
I do not get upset by a little prolapse. Kittens push really hard after diarrhea bouts. They just do. Resolve the diarrhea with the fecal check and a gradual transition to a good diet. If still having diarrhea I try panacur or metronidazole for just a few days.
The kitten tells you what to do from there. If playful and happy I manage the prolapse with belly massage and exercise. I am very very reluctant to add a purse string. Too pain and they start to push even harder.
If I have to purse string it is only for old cats. Or for puppies. But you often have to place it and then remove it 3 days later. And then replace it again. Often it takes two or three times. And no one ever wants to do it that often. So. I just try to avoid it in the first place.
Kittens heal soo fast. Just give them a chance .
Hello,
If it truly happened overnight then it is most likely infection. If he is acting normally it may not need to be addressed today. But I do recommend it be seen asap. The longer infection sits the more difficult and dangerous it is.