Could it be Rectal Prolapse?
It is probably anal prolapse. Rectal prolapse is a much more serious condition in which the entire rectal length of the intestine may come out. Do bring the kitten to the vet to have the kitten diagnosed. I have personal experience with a kitten with anal prolapse. It looked something like a bright pink “donut” around his anus which was at its worst either when he played hard or after defecating. I had to find a home for this kitten, the offspring of a stray, so I was very concerned despite reassurances from the vet ( he was seen twice before re-homing). I now forget what surgery was needed if it did not resolve but I do remember it was not a major procedure. As it turns out a vet tech adopted him anal prolapse and all. His prolapse, as it turned out, fixed itself (!) a couple of months after I re-homed him so no surgical intervention was needed. Get his poor little butt looked at by a professional as soon as you can.
