What Causes Violent Shivering Along With High Fever?

I have no other symptoms except violent shivering and a high (for me) fever of 101. I'm 88. I shoveled snow in the morning and felt some body ache later in the day. Unusual. I'm in great shape. That night I awakened at 1:am freezing and shaking violently. Never happened before. I bundled up with blankets, took two asprin and tried to go back to sleep. I last remember the time to be 3;00am so I obviously fell asleep. I awakened at 7:am with diarrhea,slight body ache but no further chills. I felt rotten all day. Unusual. I still do but much less so. No fever and no diarrhea. What could have caused this unusual reaction in me?