What Causes Sudden Lightheadedness, Weak Legs And Tremors?

I was walking during my lunch hour and started feeling very lightheaded, weak legs and thought I was going to pass out. The weather was nice about 70 here in L.A. Somehow I made it back to the office and stayed in the bathroom until I felt I could go back to the office. Got very nervous and shaky. Any suggestions as to what could have happened to me? Little over weight but that s why I m walking.