Hello Welcome in H.C.M. In reference to your query regarding headache, dizziness, and nausea. As you had noticed that even after nose drip your C.T. Scan is not showing any sinus pathology therefore most likely your headache is due to allergic nasal inflammation. Other nasal diseases giving headache are- 1- Choncha bullosa.- This is enlarged middle turbinate touching to septum. 2- Hypertrophied inf turbinate. 3- Deviated nasal septum. 4- Nasal polyp. Beside this you should also take care of other causes of headache like ophthalmic , systemic (like hypertension) and neurological involvement. So it will be better to consult any Physician for thorough examination and necessary investigations. Regards.