My husband has been experiencing the same problem for over 50 years and no doctor has ever been able to diagnose what could be causing this very perplexing problem. There is never any warning whenever he has the beginning of this problem
He will SUDDENLY experience extreme chills followed by very severe shaking (rigors)? The only thing he wants to do is go to bed, get under several blankets and wait for the shaking and extreme chills to go away. Sometimes he will develop a fever which lasts for approx. one hour. The fever then breaks and he starts to feel better.
Over the years and after many blood tests, which always come back negative, he has just about given up. The last episode was the worst ever and it took him over a week to feel somewhat normal again. All of his muscles were extremely sore due to how hard he was clenching his muscles while in bed waiting for the "episode" to subside.