Based on your symptoms, you likely have acute gastroenteritis, and dehydration is causing your headache. Your immediate priority is rehydration. Drink Oral Rehydration Salts (ORS) or electrolyte water after every loose stool. For vomiting, an antiemetic like Ondansetron (4mg) can help, and Paracetamol (500mg) can relieve your headache and abdominal pain. Avoid taking anti-diarrheal medicines like Loperamide without a prescription, as they can worsen infections. Eat light, bland foods like bananas, rice, or toast. If you cannot keep fluids down, have a high fever, or see blood in your stool, please consult a doctor immediately for evaluation.