@ AlCamus: Here is a link I found on Egg Yolk Peritonitus. The images are some of the best I've found with just a little searching. Did you find this? Did your hen look like some of these images?
http://forum.backyardpoultry.com/viewtopic.php?f=5&t=7990068
http://forum.backyardpoultry.com/viewtopic.php?f=5&t=7990068