In some cases, chronic fatigue syndrome (CFS) develops after a flu-like illness such as mononucleosis (mono) or after a period of unusual stress. But it may also occur without warning, even if you have not been sick.
The fatigue may come upon you gradually or quite suddenly. Because fatigue can be vague and can be caused by many things, you might not pay attention to the problem for several weeks or months. It is hard to say what is normal with CFS. That's because the diagnosis often is not clear for some time.