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.
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!