This is the number one culprit. When you have a sinus infection, you may have a fever, which causes sweating. You might also be breathing through your mouth because your nose is stuffed, which dries you out further. If you aren't drinking enough water because you don’t feel well, your blood volume drops. Lower blood volume means lower blood pressure, which is a primary cause of fainting.

While fainting isn't a "classic" symptom of sinusitis, the strain the infection puts on your body—specifically through dehydration and pressure changes—can certainly lead to a dizzy spell or a blackout.