I suffer from fatigue and lightheadedness for about a week at a time. It happens about every four weeks. It s enough to keep me in bed most of the day, so I am missing a lot of work. I am a 60 year old male, had 5 bypasses Memorial Day, 2014 and have participated, 2 to 3 times a week, in cardiac rehab. Blood tests are coming back fine. The cardiologist has not found anything. Once it passes, I can exercise without any problem to the point of working up a good sweat, 30-40 minutes of cardio and 15 minutes of weight training, alternating with an exercise class about half the time. Suggestions?