I had the same.problem. It's because you are are putting something so sugary into your body that you are okay for a few hours but then have a crash because you didn't have a balanced snack that could sustain you for longer. It's almost like you have a sugar rush then you are out of calories for.your body to use hence the light headed feeling. Good luck!
Sugar lifts up your blood sugar level temporarily and then it crashes, to less than it was before you ate the sweet thing. If you are eating a lot of sweets, you'll definitely experience this. Try for some snacks that have protein and more complex carbs in them, they last longer.