If you are up to it, and it's no shame if you aren't, you could perform a do-it-youself necropsy and look inside her to see what might have killed her. Two things might be obvious to an untrained person. One would be a soup of small pea-size tumors floating in her abdominal cavity. This would indicate EYP or internal laying. There may be full size eggs floating there that appear to be cooked.