and headache is due to the sinusitis
and not due to the medications.
Medications taken on empty stomach may cause pain abdomen or nausea
due to gastritis
and not dizziness.
Moreover,a bout of sinusitis is quite common after getting flu and since it is viral infection,there is never any need of antibiotics.
Only treatment required for you is steam inhalation and avoiding cold drinks.
Antibiotics are generally of no use in sinusitis and are given only if there are signs of bacterial infection.