Brisk exercise raises core body temperatures. Normally, through the process of vasodilation, the body attempts to cool itself by diverting blood flow to the skin and superficial tissues where heat can be transferred to the environment. When peripheral blood vessels dilate, blood pressure will drop unless the body can compensate by raising cardiac output. A major factor of cardiac output is volume. Dehydration is defined as a lack of volume (remember, blood is mostly water).

The symptoms you describe are most similar to a brief episode of hypotension (low blood pressure), secondary to the vasodilation associated with exercise induced temperature regulation. Plainly stated; you get hot, and your blood pressure drops too low when your body tries to cool off because you're dehydrated. As a result you feel sick, dizzy, weak, and maybe feel like you're going to "pass out."

Best practice is to keep yourself well hydrated, and cool down appropriately at the end of your ride. A light snack before your ride would be prudent. When was the last time you did a fasting blood glucose test? If you feel you're adequately hydrated yet continue to experience the described symptoms, get yourself to a doctor.