Hepatitis B question?

I currently donated blood at my high school and i got my blood test results back in the mail today. My blood couldn't be used and it says i have been tested with hepatitis B DNA positive, Hepatitis B Surface antigen (HBsAg) Confirmatory Test positive, and Antibody to Hepatitis B Core Protein (HBcAb) reactive. Im 17 and i havent had any sexual intercourse so how have i been infected with hepatitis b?