My girlfriend was recently diagnosed with hepatitis b, but my results are negative, could i be immune?

Most likely, no: Are you immune because you were vaccinated or you have evidence of previous exposure but your cleared the infection? If you were infected before you are fine. If you were vaccinated, there is a theorethical chance your antibody protection will decrease and you could possibly get infected. In this case you would need boosting of the hepatitis b vaccine. Get a hepatitis b surface antibody checked.