I meant maybe you have an infection brewing as that can cause elevated BS levels. A cold, or sore throat or water infection?
My other thought is that maybe because you had no carbs for breakfast, gender went walking that your liver just overshot on supplying the necessary glucose for exercising. Been reading some stuff online by Roy Taylor a diabetic specialist ( he of Newcastle diet fame) and he points out that one of the hallmarks of diabetes is that the liver doesn't switch off glucose release after fasting period so well and keeps on giving even when our food should be taking up the slack as it were so maybe that comes into play as well.