What Causes Stomach Pain, Headache And Chills?

I thought I had the start of the flu this past Thursday. After eating left over pizza from dinner for lunch. I had stomach issues like I was going to throw up, headache, chills. No fever though. Sick feeling in stomach for rest of day, lasting through the next day. Symptoms stopped yesterday. Only to return today. Stomach pain now, no fever, chills. What is this? Eating doesn t help. Eating nothing at all doesn t helps either.