If you do cull or lose her, if possible I would get a necropsy by your state vet to get a diagnosis. Sometimes new information comes out which can help us understand what happened. If you can’t get one, then think about doing a cursory necropsy at home to look at organs. Take pictures to post if possible. This link has state vet and poultry labs contact info:
http://www.metzerfarms.com/PoultryLabs.cfm
http://www.metzerfarms.com/PoultryLabs.cfm