It could be a number of things, be canine distemper
Found this on the internet..
CANINE DISTEMPER
While rabies are rare in ferrets, canine distemper is not and is almost 100% fatal in ferrets within 12 to 42 days after exposure. Once a ferret gets the disease, euthanasia is recommended to prevent the spread of the disease to other animals and end the animal's suffering. Some of the sign of canine distemper include loss of appetite, foul-smelling yellow or green sticky eye discharge, swollen eyelids, green or yellow nasal discharge, swelling of lips and chin, thick brown crusts tha