Try eating some protein for breakfast, sometimes the sugars in the fruit and the carbohydrates can "crash" your blood sugar.
Also, if this is the first day of your lifestyle change, then it's maybe your body saying, "Hang on a minute... we usually eat more than that! WHERE IS MY FOOD?" In which case, try to ride it out for a while, drink plenty of water, chew some sugar free gum or clean your teeth
