This explains why I sometimes feel nauseous too. I'm not overweight - my normal weight is 91lbs (6st 7oz). The other symptom I get is I get very cold and it take ages to warm up. All points to low blood sugar. I know for years, even sitting in the house doing nothing, if I get cold, if I eat something sweet I warm up within about 10 minutes.

Next time I'm out for any length of run with the bike, I'll take something sweet to eat (or drink) and see what happens.