What Causes Fatigue Along With Severe Chills?

On Dec 21st I thought I had the flu. Woke up with teeth chattering chills. I had no fever, no sore throat, no congestion, no nausea or vomiting. I had severe fatigue and slept for 5 straight days and was very thirsty. I thought I was over it but since then I have been still fatigued. What do you think this was or still is?