Give it another week or two! There is a changeover process where your muscles "readjust" to burning fat rather than sugar. Some people get headaches and tiredness but it should clear up a few weeks after starting the diet.
If you've cut a lot out of your diet there is also a risk that you're not getting some important nutrients, but seeing as you've only been on it a week I doubt this is the problem. Do be aware that you need to eat a balanced diet!