You mean you feel light headed, and faint? That is what happens when your body can not get enough oxygen or energy (simple sugars) to your brain and muscles quickly enough.
In either words:
a) you must be working out on an empty stomach, and doing too much excersie too quickly for your body to break down fats or catablize muscles to use as energy.
B) you've eaten a meal consisting of complex carbs before your workout, not enough simple sugars to use as energy.
C) you're not breathing properly. Remember, inhale on the rest part of the motion, exhale as you move the weight. So if you're doing sit-ups for example. When you move up you exhale.. And when you move down you inhale.
Also, get a simple sugar carb drink of your choice. (gatorade, carb-x, etc.) and drink that while working out to keep glucose in your body.
Justin
