Have you made any other significant changes in your daily routine apart form the vitamins? I.e. Are you excercising at a high intensity?
My first theory is that your body might be undergoing a bit of a detox process. When you make significant changes in what you're feeding your body, and how you're excercising etc, your body may go into a state of detox which will make you feel worse, rather than better - a prime indicator of this is cold/flu like symptoms. My suggestion is to persevere(!) - it can actually be a good thing, as once your body has gone through this process, it's able to make a fresh start as it has had time to clean out your internal bits, will be ready to accept the goodness your are offering it.
Of course, if your symptoms go beyond that of a regular cold, you should go to your doc to get it checked out as you don't want to be passing it off as detox if it's something more serious.
Hope the above makes some sense!
Don't believe everything you know