Thank you for your query on Healthcare Magic,
There are two reasons for rapid breathing with fever.
1. Fever, the raised temperature lead to increase metabolism
leading to increased demand for oxygen causes tachypnea
causes fever along with rapid breathing.
If rapid breathing is due to fever, it will decrease once the temperature comes down, but it doesnt happen with lower respiratory tract
infections like Pneumonia.
But physical examination is important by a doctor.
Hope I have answered your query.