I'm sorry that your question wasn't answered in a timely manner. We don't have many avian vets on this site. Unfortunately, Fluffy's lethargy and fluffing can indicate any number of illnesses or health issues in chickens. In avian medicine, there's rarely one cause of such a presentation, so we usually begin with a list of differential diagnoses and use lab tests, X-rays, and physical exams to differentiate one from another. Necropsy of a newly dead or a sacrificed severely ill bird then refrigerated (not frozen) can be an important diagnostic particularly in large flocks. If you have a large flock your best course of action is to reach out to your county-extension poultry personnel or avian-oriented veterinarian (please see here: www.aav.org) for help in arranging a necropsy.
It's best to approach the diagnostic process with a clear sense of Fluffy's financial value to your operation. Although some services such as your county animal disease diagnostic laboratory might be available free of charge through a county agency or land-grant extension office, the expense of some diagnostic tests and treatments can add up quickly. While it’s always worth your time and money to identify a bacterial or viral infection that could potentially impact more than one member of the flock, this might not be the case with a condition that only affects one hen. It frustrates me that I can't be more specific for you but such is the dilemma of poultry owners and vets alike.
Until an avian vet can take a look at Fluffy, please bring her inside as shown here: http://www.the-chicken-chick.com//search?q=sick+chicken
Please respond with additional information. Are there are other symptoms that have arisen - a change in Fluffy's eyes, sneezing, nasal discharge, increased respiratory rate, regurgitation, vomiting, swollen coelom (abdomen), lameness, weakness/paralysis in a limb, wings, or neck, or squatting frequently as if to lay and walking like a penguin?