I'm so sorry to hear that...was it a kit born to one of your bunnies (I wouldn't think so if you had it quarantined)?? How old was it?
Bunnies do go downhill fast by the time you are able to realize there is a problem. I figure it is because they are a prey animal with little to no defenses, so they are stoic, so as not to show weakness to predators.
Kristen