Taking antibiotic for sinus infection for 3 days but not much improvement. my fever keeps going up between doses like its not strong enough?

Not too surprising: In truth the vast majority of sinus infections are caused by viruses which are not affected by antibiotics and resolve only with time. Since your doctor felt an antibiotic was needed, you must have shown some signs of bacterial infection, but even so it can take 10-21 (yes, up to 21!) days for symptoms to clear up. As long as it's not worsening actively, it's usually best to wait and see.