What Does Severe Fatigue And Fainting?

My sister is having a health problem that's unidentified at this point. Besides a feeling of unwellness all of the time, she is severely fatigued and weakened to the point of passing out just sweeping the floor. Numerous cardiac causes have been ruled out. I'm trying to research online what could cause this problem. I was just going into Addisons disease when this consult page came up. Do you have any thoughts, with such skimpy information?