Update 2: This morning, Big Bird died suddenly. After about two months of not laying and looking pretty ill, last week her comb had gotten a lot more red, her feathers had become softer, and she had started laying very healthy eggs; she even layed one yesterday. I thought she had gotten better and was just going to keep getting better. But, as much as I hate to admit, she probably had never completely recovered from her mystery illness, as she still retained some signs of weakness, like early roosting and being the last one to come out in the mornings. I had started feeding her some wet chick starter in the mornings while she was on the roost to make sure she was getting enough nutrients, because when she was at her worst she sometimes would stay in the coop up to two hours after I would let our other chickens out, and it seemed to help her. As for our other chickens, they seem to be healthy and are not showing any visible signs of illness, but I'm still very nervous that one of them will meet the same fate. We cleaned out the coop right after we found her dead and are planning on burying her sometime this weekend, but I am convinced we should do some kind of necropsy.
We've had very bad luck with all of the buff orpingtons we've had, and she was the last one to go. We started out with three; one of them died at around two weeks old from a respiratory infection, the second turned out to be a rooster and had to be rehomed because of our township's policy, and now Big Bird is gone at only 11 months old. I really don't know what happened, it could have been an illness, poison, or some kind of heart attack.
I'm currently very emotional, so I'm sorry about the dump of information I just gave, I just really needed to let some of my emotions out one way or another. I really hope something like this doesn't happen again anytime soon, so I'll continue to work hard to make sure the rest of our chickens stay healthy.