How was your hydration up to and during your run? What about eating? It's difficult to say with all certainty. It could be that you were suffering from a drop in blood sugar (this can happen if you run too fast--because the body uses a much greater percentage of glycogen) if you had not eaten in a while or if you were dehydrated. You may have to try timing your eating better before your runs, however, if this remains an issue you may want to talk with your doctor about this.