hepatitis b even though i was vaccinated?

I am REALLY confused. My doctor called me up saying that my blood tests show that i may have hepatitis b, but I was vaccinated! I also ALWAYS have protected sex with my boyfriend who has been my only partner. She thinks based on the circumstances that it's highly unlikely that I do have it, but wants to do another test. Does this make any sense to anyone? Why would I have a false positive? Is there really any chance I could have contracted hepatitis?