Numbness in extremities is usually caused by decreased blood flow (or oxygen flow), usually because of improper equipment, your equipment being too tight, or maybe even something wrong with your health. Numbness can also be caused by fatigue, so maybe try toning it down a bit when you are cycling, and see if it helps. Don't push yourself too much, as it would lead to wear and tear of your body parts and would definitely mess you up in the long run.