On some occasions, I get similar symptoms if I eat something sugary within an hour of riding. Seems to screw with my blood sugar levels and insulin response. Try eating a low GI carb meal more than 90 minutes before you start. If you're riding early morning, just eat a good meal the night before and bring ~ 250 calories/hr and forget the pre-ride meal (but start fueling frequently and immediately after you start riding). Seems to work for me.