The information limited with just fever and no other symptoms. The source of infection seems unclear. Usually there will associated symptoms like throat ache ,cold, fatigue, body pains, joint pains, cough, loss of appetite, abdominal pain, nausea,vomiting, Diarrhea, burning urine. Recheck if any of these symptoms are there to determine source of infection.
Immediately consult a general physician.
Since the covid test is negative get a
Complete blood count, complete urine examination,
CRP done