That is only a guess that they ate anything toxic. Have you tried to get a fecal test on some droppings yet? A necropsy on the body would have been the best thing to have done. How are the other chickens feeling?
I would look around for any dead animals or vegetation that has been underground or underwater. Botulism might be a possibility. Signs are flaccid paralysis that starts in the feet and moves upward in the body to wings, neck, and eyelids. Death occurs within a day. Many things can look similar...
So sorry that you lost your hen. The best way to know what killed her is to get a necropsy done. Where is your general location? Can you save the body and keep it cool wrapped in garbage bags, so that the state poultry vet could do a necropsy? You can call them early tomorrow for rate and...
Sorry about your sick hens. Their symptoms sound a bit like reproductive disorders, but you say they are both still laying? Are the eggs normal with strong shells? I would look them over for mites and lice. How do their crops in the right upper chest feel? Are they empty and flat, full and hard...