I currently have a Green Tree Python that is about 3 feet long and still has the yellow juvenile colors. The cage is kept at about 55-60% humidity and a temperature of about 85° F. The snake's cloaca vent is currently swollen although there is no prolapse protruding from the vent. The snake acts perfectly fine. Although I have experience with prolapsing, I have never seen anything like this before. The cloaca looks perfectly normal (normal coloring, nothing protruding, etc). It is just swollen.
I feed the snake, in my opinion, undersized hopper mice. The swelling has been there for a couple of days after I have fed it.
I apologize for the lack of pictures. I know it's difficult to make a judgement without pictures; however, I hope you understand due to the not so friendly nature of the snake.
EDIT: I know it's very difficult to see but here is a picture. The opening is the all black marking on her underside. The black is part of the natural coloring for the snake. The swelling is difficult to tell from the picture though.