What Causes Persisting Fever?

I am a 39 year old female that had my tonsiles, adenoids, uvula surgery and was removed on March 15. Also, there was nasal obstruction and trubinates added to my nose. I have had a temp from March 16th but my ENT doctor said I didn't have an infection. I went back to work 3 weeks. Tuesday morning I woke up with a fever of 101.8 went to Primary he said I had a URI. I am still running a fever and it's amost a week later of 100.3. Please help what's wrong!