Why does my body shut down after exercise sometimes?

Say I really push myself (mostly at the gym): I’ll be doing my exercises, getting my gain on. I’m pretty adept at listening to my body’s cues (drama school will do that), but maybe 5 minutes after I feel like I’ve had enough, I’ll get waves of feeling horrible.

Think wretched. I turn visibly pale/green. I’ve had episodes where I start losing vision on my periphery and sound becomes muffled. I’ve thrown up once. I get clammy. Pretty much the only thing that gets me out of the stupor is lying on my back for about 20 minutes. What is UP?

Also, I think it has to do with the type of exercise… I’ve never had a problem playing 3 back-to-back racquetball games for over an hour. I’ve noticed that large muscle exercises (especially leg lifts) cause the symptoms more.

I’ve always had low blood pressure and low body temperature, in case that’s something. My lungs are pretty good. Apparently I have pretty high tolerance for heart-related things (or at least I did 5 years ago when the nurse said “I’ve never seen anyone run for that long with their heart rate so high”, though looking back, I felt horrible after that too).

Any thoughts? Is this a normal thing?