I get dizZy, lightheaded, and week?

I could be out on the tramp, jumping, on the swings, running around. I will be just fine before these activities and durring these activities but, soon I begin to get a little dizzy, and lightheaded, plus really weak and even shaky. One time my dad had me some chocolate, then after a couple of minutes I was just fine. What does this mean, what do I have?