Just to let you know, its nearly impossible that you will experience any nutritional deficiency from 3 weeks of veganism. However, if you are not eating enough, you could experience feelings of weakness, headaches, and see "stars" like you describe (from low blood sugar.)
Please read around in the health forums. You will see a lot of posts like this. Please note, however, that many of these questions are from people with eating disorders, not people with healthy vegetarian diets.
In the meantime, please get a book called "Becoming Vegan". It is written by nutritionists and will calm some of your fears about your new diet.