HI, Brendan.
 
You don't say how old she is.
 
If she is middle aged to older and has lethargy some things your vet
will check for are blockage, heart health, and blood glucose levels
(insulinoma), and some other things.
 
If she is a kit then blockage will be checked for as well as some other
things.
 
Lethargy is a pretty general symptom, but what may be a fail attempt to
pass feces without success may be a pointer to the cause.
 
Is she able to pass stools at all?  Is she eating and drinking?  If
any of these stop you need to get an emergency appointment right way;
blockages can be rapidly fatal because of blood toxin build-ups.  When
tehre is a blockage care can not wait.  If her stools have become
unusually thin then that also can indicate a blockage - - just not a
complete one.  Watch, too, for fever or from black or dark green tarry
stools (meaning digested blood), or for frank blood (red).  A blockage
can sometimes cause a perforation.
 
The most likely things to cause blockages in ferrets (cloth, rubber,
latex, some dried veggies or fruits, raw hide, etc.) tend to not show
up on x-ray, though sometimes the related gas build-up will.
[Posted in FML issue 4348]