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It sounds like your kitten has a viral infection. These are commonly the cause of fever and lethargy. They will typically run their course over 5-7 days. The fever can be intermittent, leading to lethargy and decreased appetite. I usually treat with fluids, antibiotics and an anti-inflammatory med. The fluids help with hydration. The antibiotics help in case there is a secondary bacterial infection. The anti-inflammatory (Metacam usually) helps to bring the temp down. Once the fever breaks, they start acting much more normal.
Keep trying to get fluids into her. I would also try to discuss antibiotics or anti-inflammatory meds with your vet to see if that helps.
I hope this helps, let me know if you have any other questions.