hello i'm 29 and faint very often from gym work out after relatively little exertion.its a slow build up to fainting.usually feel dizzy/weak become drained of colour for 10 minute before eventually blacking out.had my resting blood pressure taken and its normal.but this things bothering me.lately ive felt like weird twinge in my back that rattles a split second after my heart beat.my dr gave me a resting ECG.which was ok.sometimes this fainting feeling is avoided if i pass motion.anyone think i've got a circulation problem? my dr dont seem bothered either way.so it looks like its down to me to investigate.any advice would be apreciated

Buy or borrow somebody's BP tester and get several readings over the course of several days.
You'll soon know whether BP is involved in the near faints.
And then go from there.

Could you possibly be drinking too much water?

http://www.nytimes.com/2005/04/14/health/14water.html

Another possibility is something called neurally mediated hypotension- do a web search on that.

But the obvious thing to do in your situation is to schedule a stress test with a cardiologist- that should give you the answer.