Sounds like it could be low blood glucose. As noted above, if you aren't eating adequately this can happen. There could be other reasons as well. Try eating a small piece of cheese or something prior to exercise and see if it helps.
It might be a reaction to the way you are eating. (If you aren't getting a balanced diet, or are eating too little, or not enough protein, or too much caffeine, or are taking diet pills, you can have this reaction.)