I've read similar stories. And.... or course hydrating was just my first thought. The OP's doctor may have a completely different take on the "light headed" part... that I overlooked.

I've heard of guys "cutting back" on fluids before cycling.... because of an unavailability of restrooms. Some people have difficulty handling a water bottle while cycling. Older guys... sometime worry about having to pee too easily. Maybe... I should have included a link:

http://www.h4hinitiative.com/tools/urine-colour-chart