Chicken with prolapse?

WE have just had to cull our boss bird, she was fine yesterday, but when I went to check on them earlier, she had a swelling coming from her rear end, it looked a little smaller than an egg size but upon closer inspection there was no way the lump could have been got into to release the egg, if that is what is was.  She was trying to poo, but just able to open her mouth and make a slight squeaking noise.. and was clearly in discomfort, although she did still manage to run when we tried to catch her!
Has anyone else had a similar problem with any of their birds and is anything in particular likely to cause it or is it one of those things?

WE are all feeling very sad now...

Thank you joyfull - I searched prolapse but nothing came up..