Ok, to be honest, I cannot tell you why. I wish i could. I would be guessing. in fact you could do just as well if you do a google search for "fatigue
" and read on it.
I mean, if your doctor who examined you, did tests, has all the results cannot tell you, it would not be fair for me to mislead you that I could come up with a magical answer.
I dont know what tests he did. I mean there are specific tests and subgroups of tests that he may not have ordered. Advanced testing might give us an answer.
Did he check for rheumatologic disease such as LUPUS, did he check for HIV? Did he do a workup on your heart? How about oxygenation of your lungs
? Do you snore at night, do you have sleep apnea??? That is a common and often overlooked diagnosis.
So you can see... this is virtually impossible. Maybe a second opinion would be worthwhile. Find another doc, maybe one who is recommended by a friend. When you go bring all your records, labs , xray reports etc.