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]