Sorry for your loss. I lost a chicken in her first year of life of no apparent reason. If you do a home necropsy on her body, if you can take a few pictures of the organs, especially the liver and major organs, if there is a lot of fat, any yellow fluid that comes out of the abdomen when it is cut open, to post here we could offer some thoughts. Most state vets will do a thorough necropsy if you send the body in, but it must be kept cold. Here is a video of a more complicated necropsy, but the organs are labeled which is helpful.