On Satuday I went to the gym. I just started working out again since 2years. I walked/ran for 20mins. Lifted some weights. I had been working out for 45mins when I felt dizzy, I felt like if I was going to faint. I looked pale, and felt weak. I was working out fine. I wasn't rushing and I was going in a slow pace. Why do you think this happened to me? What can I do to prevent this?

Did you eat beforehand? It sounds as though you may have had a low sugar level.

***EDIT****

What is your blood pressure like? This could have been something called orthostatic hypotension, which is low blood pressure when changing positions like from sitting to standing or laying to sitting.

Yes I had a bowl of cereal (cinnamon toast crunch).
Last I checked my blood pressure was fine. I had a problem in Sept (with my kidney & Liver) And also I didn't have enough potasium, so I was taking some potasium tablets. My blood pressure was high then.

Hmmm, are you still taking potassium?

Not any more. It was a prescription. No refills. And they were huge. :)

I know I know there is sooo many things wrong with me. LOL :)

LOL, nah not a lot of things wrong with you. BELIEVE ME!!

This could have just been something transient. Maybe your sugar was just a little low, could have had a little blood pressure stuff going on from the exercise and all.

If it does continue though you would be well served to make a visit to your doctor.