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.)
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.