Is it possible to have a sinus infection with horrible headaches that doesn't show up on CT scans and if so how could it be detected?
If the headaches are as horrible as you say, no. When the sinuses are infected they produce mucus which creates the pressure which creates the headaches. On xrays (or CTs) normal sinuses look dark gray or black, when there is an infection it is white. Very hard to mistake. Headaches have to be from something else. Keep looking.