I had one a few years back when I first started with my girls. Did not realize how serious it was till it was too late. For me it ended up being gapeworm which is treatable and easy to diagnose.
http://www.poultrydvm.com/condition/gapeworms
https://www.backyardchickens.com/threadloom/search?query=gapeworm&tab=618
http://www.poultrydvm.com/condition/gapeworms
https://www.backyardchickens.com/threadloom/search?query=gapeworm&tab=618