I get this as well, especially when I'm regularly exercising. In High school when i ran cross country my doc linked a combo of lower blood pressure which is natural if you're fit, and not too much you can do about, coupled with a possible temporary low blood sugar from exercising/improper diet (ahem high schooler's typical diet...) I still get it, but not nearly as much as long as I replenish electrolytes and make sure I get enough carbs early in the day to stabilize blood sugar, and fruits to help the sharp drops if i was just exercising or if i was siting for a while. I'm not diabetic, but from what i understand blood pressure and sugar levels can work in tandem to do stuff like this. for refrence I'm 23, and a pretty average build