yep, thats carb flu alright! stick with it, believe me it gets so much better.
Try eating some protein or fat and the energy levels will soar ( i know i got it wrong yesterday and didnt realise til one of my students mothers offered me some walnuts as a snack due to the fatigue) It made beginner trumpet much easier to teach!
have you seen the calling all uk groks thread? there is also a london group you might want to join