I have a 5 yo RIR hen that decided to sleep in the nesting box tonight-very unlike her and she looks like she is trying to lay an egg -she has a history of shelless eggs. She also has her wings out a bit like she's hot. Otherwise she looks totally fine, is receptive to me and has not had any symptoms -she's been totally healthy and acting normal. Her sister died a few months ago (in the winter- we're in ma) and was also totally fine until i found her sleeping in the nesting box one night as well then she got gradually worse over a few days and passed. Can they be totally fine and then just start going downhill that fast? With no symptoms?