Yellow fluid in her belly is a sign of water belly (ascites.) Water belly can occur for a variety of conditions, including cancer, fatty liver disease, egg yolk peritonitis, and heart failure. Did you see a lot of fat? Did her liver look normal and red? I will look at any pictures you post.
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...