I have had sinus infections since thanksgiving, had CT scan, and was tested for allergies. Everything was normal. Why do they keep coming back?

Sinus infection. Depends, you say you had a ct scan and it was normal. If you had a ct of the sinuses that showed no infection then you do not have a sinus infection. Possibly you have recurrent viral infections which cause nasal and respiratory symptoms. It is not unusual to have one or more viral infections per month in winter. Perhaps you are prone to these due to exposures at home or work or to children.