Hi there! Thanks for using PerCoach! There are many things that can cause a cat to have a fever and these include pancreatitis, pneumonia, FIP, systemic infections, foreign body ingestion and septic peritonitis, among many others. I advise that you have Mia evaluated by your veterinarian as some of these conditions can be managed to try to get Mia to feel better.
At the very least, Mia can be started on symptomatic therapy for now until your veterinarian is able to figure out what is wrong with her. Further diagnostics are likely to include bloodwork, urinalysis, thoracic radiographs and abdominal ultrasound. Good luck! Hoping for a speedy recovery!