Your body isn't used to these changes. Part of it will be cravings you aren't used to having for fattie foods, which will make your brain tired. Pretty soon you'll start feeling much better. Keep at it, it's a good thing what you're doing. It's just your body adjusting, it will take some time but all worth it in the end.
It's simply because you don't consume enough calories for your system to function properly. You need fat and carbohydrates, which are the main source of energy for your system. You need sugars and fat meats in your diet.
You could be detoxing. I had symptoms for a month after cutting all refined sugar out of my diet. You also might want to check this article out: Think Fat-Free Milk is Healthy? 6 Secrets You Don’t Know About Skim.