More than likely, the sick one was exposed to a respiratory disease at the same time as the flockmates. Respiratory diseases are hard to distinguish from one another since symptoms can be similar, but can be caused by virus, bacteria, or fungus. Most of those except fungus, can make the flock...
If she is fine now, I would put her back, but watch for any pecking or bullying. A dog crate with food and water inside the coop is a good way to get her back slowly if they attack her.