I have this a lot, too. I tend to believe it's Small Bowel Bacteria Overgrowth because I ALWAYS feel better when they put me on an intestinal antibiotic, like Flagyl.
Diagnosed in October, 2004 at age 33.
Currently taking Colazal, Nexium, Ferrous Sulfate, One-A-Day multivitamin, Omega-3 Fish Oil, Vitamin D, Vitamin A
Secondary conditions: mouth ulcers, joint pain, extreme fatigue