What Causes Dizziness, Lightheaded And Blurred Vision?

recently was working as a lifeguard at a pool and felt dizzy light headed sick to my stomach and i started having blurred vision. At that point I remember sitting on a nearby bench where i was for a couple min when I finally regained consciousness. I was told that i had "passed out" with my eyes open and was shaking slightly and that I had my arms slightly stiffened to one side of my body. I have felt the symptoms prior to my unconsciousness before and have even had the same reaction two years ago. What could be the cause?