I'll just about guarantee the issues have been dietary... I went through a period of stomach (and weight) trouble because I was getting a bit older, my metabolism had changed and I was still eating like I was a teen.
Started eating a little more carefully, and exercising (a LOT more) and feel far better. I'm actually gaining weight again, but this time, it's not fat (really exercising a LOT more).
None-the-less, it's always good to eliminate all of the evil things that could be wrong with your health, if for no other reason than to get them off your mind.
Now - GET BACK TO WORK! (whip, snap...)