May not be the case for you, but about 15 yrs ago I briefly lost the use of my legs. Was standing on a flat grassy patch talking to a friend and I just fell down, for no reason. Turned out I had an as-yet undiagnosed heart problem that later required surgery.
Are you seeing your doctor anytime soon? I'd imagine that stress can cause temporary lightheadedness, but so can much more serious issues that might show up only after exertion. It was problems after exertion that caused me to see my doc and find out I needed surgery immediately.