Headache, diarrhea and weakness are related to acute gastroenteritis and gastrointestinal flu. Acute gastroenteritis can be triggered by the damage of the gastrointestinal mucosa when overeating or eating unclean food, which can cause nausea, vomiting, abdominal pain, and diarrhea. In severe cases, some people may become dehydrated, and once they become dehydrated, they may experience dizziness, headache, and weakness. Gastrointestinal flu is caused by coxsackie virus infection, which causes not only digestive symptoms but also cold symptoms, so headache and weakness may occur along with diarrhea.