What Does Low Grade Fever Along With Sever Fatigue Indicate?

I have been sick for over twelve days with what I thought was mono, which I ve had before. Blood tests were negative, and my Dr thought I might have a sinus infection so I ve been put on antibiotics. My only symptoms are a constant low fever, weakness and severe fatigue. I ve been in med most of the entire time I ve been sick. With all that: How many days should I be on these antibiotics- with no change in how I feel- before I ought to call my Doctor back? what else could I have? I m gluten intolerant and careful about that, have PCOS with NAFLD and non-insulin resistant, and I m not overweight.