i had the same thing. i was taking a multi, b complex, calcium, e, etc etc. i started getting dizzy all the time. i thought it was because i was working out too much.
i stopped taking all the vitamins and switched to this supplement called Sam-e. my mom told me about that it helps your liver function better and since it's already something your body produces... it didn't harm you. so i took that and i've been better ever since. it was nice going from taking a handful of vitamins twice a day to 4 pills a day. i felt a million times better too. =)