Nausea may arise from headache, and this may arise from mayny reasons, including high blood pressure. I gusess you already have some head CT or MRi tests.
Fructose malabsorption causes nausea, bloating, diarrhea, headache, depression...
I don't think I was of much help...Can you describe your headache exactly? What happened 18 months ago? Pain is worse on touching any particular spot? Pain is on both sides of the head? Exercise, meals, sleeping - how this affect the pain? Any medication helps?
Any other gastro symptoms except nausea?