Can a pregnancy test be wrong?

me and my girlfriend had sexual intercourse and i took her virginity. so the day after she called and said she was throwing up and feeling sick all day and bled out of her part a little bit. so she thought she was pregnant but i din't ejaculate inside of her. so 13 days after she called and said she took a pregnancy test. but she sounded calm when she told me it but she said it had came out positive. does anyone think it was wrong?