John,
I would be very concerned that your ferret has a partial blockage.  We had
very similar symptoms with Daisy a couple of months ago and found she had
eaten a foam earplug which was in her stomach, rather than her digestive
tract.  It allowed her to eat and poop but was making her sick.  Please
take her to the vet immediately and have her checked (probably barium
x-ray).  It will help you to stop worrying and may save her life.
 
Karla
Meep and Daisy
[Posted in FML issue 2447]