Hi, I have gone through your medical history carefully and understood your concerns. Diarrhea, headache and nausea are indicating an acute infection of the intestines (most probably a viral gastroenteritis). Diarrhea is associated with loss of fluids and electrolytes leading to hypotension and subsequent headache and racing heart while sitting. It is very important to get plenty of fluids especially plain water and replacement therapy with ORS (oral rehydration salts) to replace the lost fluids. I also advise to take: - probiotics to stabilize intestinal flora - tasectan or pepto bismol to stop diarrhea - BRAT diet (banana+rice+apple sauce+toast) If no improvement, should consult with your physician further as might need diagnostic tests and prescription meds. Wish fast recovery.