It could be anything and/or a mixture of a number of things.
I agree with Christina. If the level of fatigue is new to you, please be sure there is no physical explanation. Have a physical with comprehensive lab work.
Closertothemid mentions sleep apnea. Another important idea.
Like Christina, I have chronic pain and fatiguing illnesses.
If you are looking for the specific "Chronic Fatigue Syndrome" criteria and are in the U.S., search under SEID, as that is the newer name in the U.S., with updated diagnostic info. SEID = Systemic Exertional Intolerance Disorder.
I hope you can identify any factors contributing to your fatigue... soon.

WC