This is my 3rd day on the zpack for sinus infection. i feel almost normal, but will get a low grade fever by mid day. why & when i will i feel 100%?

Diagnosis?: First Z-pack if not an appropriate antibiotic for sinusitis. Second, without a CAT scan of the sinuses, the diagnosis is not specific. Third, all sinus like infections start in the nose and are therefore viral, at least for the first 5 days. Depending on the etiology, you should recover in 10-14 days. If not better, see an ENT doctor.