flatulence/ distention accompanying fatigue?

I am mid twenties in age.  For the last 10 - 12 years I have suffered from daily onsets of fatigue and flatulence/ distention.  I am very flatulent, almost independent of diet.  The fatigue is sometimes profound and can cause temporary noticeable cognitive impairment.  It is also quite different from the sleepiness one may experience at the end of a long day or after having gotten inadequate sleep.  

******The two symptoms exhibit an unusual pattern:  they always accompany each other.  That is to say, never does one occur without the other also occurring.  So, if the GI symptoms are present, then fatigue symptoms are present.  And if fatigue symptoms are present, then the GI symptoms are present.  This holds true also for stages of onset and passing.******

I am in quite good health, both physically and psychologically.  I have been tested very extensively for GI disorders.  Some observations of abnormal tissue colouration or motility have been noted, but never with sufficient evidence for a reasonable diagnosis.

What troubles me:

The biochemical etiology of this is a complete mystery.  What is causing the fatigue?  And how is it related to flatulence and/ or distention?

Further, I have never met nor heard of anyone who has suffered from this.  Any thoughts?