Hello and thank you for this important question about your little one's health.
A ferret of this age is a candidate for having a type of tumor called an Insulinoma. It causes her body to secrete extra insulin that then accelerated the processing of blood sugars...she is then weak all the time. The beginning stages of this disease can come and go, so she can appear normal inbetween bouts of decline.
Most of the time, these tumors are malignant ("bad"). A veterinarian's examination is the only way to confirm this...there are cases of ferrets with diabetes as well, but this requires medical assistance too.
This is a link to an article about insulinomas in ferrets, it also links out to a number of other health-related topics you can investigate.
http://exoticpets.about.com/cs/ferrets/a/ferrinsulinoma.htm
Here are some sites that advertise a ferret vet index (if your vet is game to continue treatments with you, then he/she may be able to get a consultation by phone with an experienced vet):
http://www.quincyweb.net/quincy/vet.html
http://www.ferret-universe.com/vets/vetlist.asp
http://www.ferretsanonymous.com/vetlist.html
http://www.ferretnews.org/clinic.html
http://www.ferretclub.com/vets/
http://www.rabbitrodentferret.org/rabbitrodentferret.org/vets.asp?animal=3
I hope that this information at least points you toward the answers you seek, and I hope that you can contact a vet in your area who can help you.
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