What Causes Fainting Sensation, Heart Palpitations And Fatigue?

Hi, I have been feeling faint the past two months. There were times when I was very fatigued. I have had heart palpitations. My iron level was 46 mg 2 months ago, and 60 mg now. I have no history of heart disease and cancer in my family. I used to exercise about 2 hours of cardio six days a week in 2014. My doctors are at a loss. ECG, echocardiogram, Holter all show nothing.