help diagnosing cause of death

I am broken-hearted that my and my husband's favorite hen, our baby, died overnight. It's a long story why she's our favorite, but the short story is she's a product of our flock, but no one wanted to mother her, so I raised her from Day 2! For a variety of reasons, we named her "Supergirl." I'd like to determine the cause of her death, partly so I know whether it might be contagious.

Supergirl was just short of 5 months old. I'm pretty sure she wasn't laying yet. I didn't interact with my 30 chickens, who free-range all day, yesterday until afternoon, so I didn't see her till then. She didn't come running when I gave them treats, so I went looking for her. She was all fluffed up and completely uninterested in food or water, which I've NEVER seen in her. She was able to hop up 2 feet into the coop, then got up on on their roosting poles 5+ feet off the coop floor. She was very lethargic and wanted to just stand, all puffed up, looking like she was falling asleep (both eyes closed most of the time).

I came in and did some research, then went to get her (she did put up a bit of a fight when I picked her up, but not like usual). She's usually pretty chatty but didn't say anything. I took her into my office (an outbuilding), gave her a dropper of olive oil, and massaged all over her chest and belly for about 15 minutes. I didn't feel any bound eggs or anything else unusual. She sat in my lap quietly, eyes closed, but did open her eyes and look around when she heard sudden noises or the other chickens squawking.

I left her overnight in my office, with a bowl of water. This morning, I went in and found a trail of 6 or 8 watery, white droppings (looks like white-ish diarrhea) leading to her body, and a big pool of what looks like water around her head.

Any idea of what this might be?? She is one of only 3 that we've hatched; the others all came two years ago from a hatchery and were vaccinated for the standard diseases, but she and the other two hatchlings have not been. Everyone else seems just fine this morning.

Thank you so much for any help!