What Causes Headache, Nausea And Fatigue?

I started not feeling well on Monday. It started as a headache and then nausea set in. I started to feel fatigued and achy. At first, I thought I was getting the flu. I had the chills one minute and couldn't get warm. Then the next minute, I was perspiring. I have also had a low grade fever. The nausea and headache comes and goes. Today, I have had severe stomach pains, diarrhea, and a great deal of gas. I have never had this set of symptoms at one time. I have tried Advil, Pepto-Bismol, Imodium, and Tums.