Why is my body tensing up at random times in the day?

Up and down BS?: Intermittant episodes of low blood sugar will cause bursts of adrenaline as a result of sympathetic nervous system stimulation. This could cause your body to tense up. This symptom pattern is called reactive hypoglycemia & is the result of low fat, high carb diets. Stop sugar, eat meat or cheese or fish or eggs with each meal. The healthy fats acts as "log on metabolic fire" to keep sugar stable.