you could simply be having hypo symptoms. i bet you aren't eating enough or you are trying to do the keto diet. one of the symptoms of being in ketosis is chills. this is why i am not a fan of this low carbs bs that all the young kids are doing these days. they think that starving their bodies like crackheads is good. carbs are not the enemy, eat some oatmeal, rice or sweet potato before your workout