Sinus Medication Solution

I am 28 year old female. I am 5′7″ and weigh 130 lbs. I have a year old script for some bactrim and can’t remember how many mg I should take for a sinus infection. They are 800 mg pills.

I am currently taking Bactrim SMZ/TMP for a sinus infection, i`ve been taking it for the past 3 day, and it feels like the infection is gone, but the Backtrim is causing me to get really bad nausea and dizziness.
Is there another anti-biotic I can switch to or is there any way to aviod the side-effects of the bactrim?