Skinny ferrets

Question
I have 3 ferrets that have gotten very skinny in the past few months, I have had them for about 3 years and the only thing different is I havent let them out of their cage because we put out mouse traps because of a mouse problem. I didnt think that would cause them to get so skinny but maybe I'm wrong. We have recently gotten rid of the mouse traps and let them out again but they dont seem to be gaining any weight. Any theories you may have I'd like to hear. Thank you.  

Answer
Are your ferrets 3 years old? Or were they older and you've had them for three years?  Ferrets can get stressed out and can lose weight.  If they were used to being out and then were caged they may have gotten stressed but they should get over it if they are out and about now.  Have they had hair loss or any other symptoms?

Mary