I'm sorry that your question wasn't answered in a timely manner. We don't have many avian vets on this site.
Unfortunately, the symptoms you've mentioned - lethargy/listlessness/inappetence can indicate any number of illnesses or health issues. In avian medicine, there's rarely one cause of a condition, so we usually begin with a list of differential diagnoses and use lab tests, X-rays, and physical exams to differentiate one from another. With this in mind, 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 differentiating the various causes of what you're seeing. Veterinarians can perform a physical exam and run diagnostic tests, including X-rays, to distinguish between the various etiologies.
It's best to approach the diagnostic process with a clear sense of Tipper'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.
Yes, isolating her for now is smart. We don't have much of a track record on her because she's acting ill for less than 24 hours. I need you to re-evaluate her in the morning and then return to our conversation with an update. I need to know if any new symptoms arise - conjunctivitis, sneezing, nasal discharge, coughing, increased respiratory rate, regurgitation/vomiting, diarrhea, or lameness. Please tell me how old Tipper is in weeks or months. Please respond with further questions or concerns if you wish.