What Causes Fever With Sinus Infection?

WHAT MAKES YOU RUN A LOW GRADE FEVER THE DR STARTED ME BACK ON A Z- PACK. i'D REALLY LIKE TO KNOW WHY I'M RUNNING ONE I HAD bLADDER SURGERY. BUT THIS STARTED OUT AS MY SINUS I LOST MY VOICE WHEN I STARTED FEELING BETTER I ASUMED I WAS OK NOW I'M RUNNING IT AGAIN AND FEEL AWFUL