I had one do a very similar thing the other day. The more I tried to see what the matter, the worse it got. I thought maybe the vent was plugged internally as the abdomen was full and round feeling. After he passed I did an autopsy (an old labor&delivery/er/surgical nurse). I found his heart was bigger than his liver and everything was being crushed by the pericarditis (fluid around its heart. This was my first and I hope my last. Nothing you can do for the most part except treat with antibiotics and catch it in time, still not guarentee.
Sory for your pain and the chicks
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