S.R.
My now 4 yr old did the same thing around the same age. I was freaking out thinking he had a neurological problem. I did some research and found out he was getting the chills after, or during he was peeing. It happens to adults. Check next time to see if he had just gone to the bathroom and if he is wet, that could be your answer.You should go and ask your Dr. I feel for you. I was so scared when I saw my son TWITCH. Then when I finally realized what was happening, he stopped getting the chills! Good luck!
~S.