What Causes Lightheaded , Fatigue And Weakness?

I am a 42 year old male,6 ft with weight of 185. I am in good health, recent blood work up was normal, am on low dose of BP meds...20mg of benacar 1x day. Back in Feb had episode of light-headedness,thought might pass out and fatigue and weakness. Call Ambualnce came and ckd bp:normal,ekg:normal, pulse elevated at 120(was very anxious also)....after about 20 min. started to go away. Was fine until few days ago, had another short ep. of light-headed and weak, went away after few min. Scheduled for stress test...any ideas or suggestions?