Relle, the thing that springs to mind for me is blood sugar levels.....(I am no expert BTW!) but after Lexie, I really needed to up my intake of low GI foods, to maintain energy. If I was slack, I too noticed the fatigue and the dizzy spells.......(I was OK as long as I had In-Liven & a low gi diet and LOADS of water.)

But like bath suggests, it could be anything. (Maybe get yourself checked JIC?)