We ended up putting her down

I really didn't want to, but felt terrible watching her suffer. So far, nobody else in the flock is exhibiting any of the same symptoms. To answer a few questions, we are using pine shavings as bedding, and I change it every couple days. They are in a very large stock tank in my garage. While it is probably drafty at times (when I open garage door, etc.) it doesn't stay that way for more than a few minutes. Based on the entire flock's behavior, they're warm enough. I've only been feeding medicated chick starter, with a handful of dried meal worms once a day. Once the sick chick was brought inside, she was definitely kept warm enough & had no drafts, was kept on paper towels with same feed. No problems with sick chick (or any other chick's) vents, ears, eyes, etc. They don't have access to anything that could be spoiled/molded. I'm bummed I don't know what caused the problem, but hoping it was an isolated incident. I'm completely reaching here, but other people having this issue, do you have styrofoam in your brooder? The stock tank mine are in is HUGE, so I have them sectioned off with a piece of styrofoam. They, of course, peck it like crazy. I've made provisions to stop the issue, but I thought maybe sick chick got some sort of .... whatever... from ingesting styrofoam???