I am no expert, but I try to research as much as possible. Please read the info on these links because it sounds very similar to impacted crop.
http://fresh-eggs-daily.blogspot.com/2012/02/sour-crop.html
http://www.the-chicken-chick.com/search?q=crop
http://fresh-eggs-daily.blogspot.com/2012/02/sour-crop.html
http://www.the-chicken-chick.com/search?q=crop