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Hello. Thanks for the question.
In a young cat like this, with lethargy and a fever, it sounds like it's some type of upper respiratory infection. It doesn't sound like she has the sneezing or ocular discharge signs. It may be that she has more of a laryngitis in the throat as opposed to the eyes/ nose.
Most of these are infections that will run their course over 7-10 days. If they stay hydrated and continue to eat, they usually resolve on their own. I would just try hard to get fluids and food into her. A canned food warmed up in the microwave would work well.
If she seems to be worsening, then I would have her seen and potentially run bloodwork to make sure her organ function is normal. They could also treat with some antibiotics and fluids to help with her hydration. It does sound fairly mild to me, though, and I bet it resolves on its own.
I hope this helps, let me know if you have any other questions.