Group b strep swab....

So I gave birth in 2004 to my first daughter, rylyn, I was swabbed for group B strep, I was positive and had to get antibiotics during labor....I'm 9 months pregnant with my second lil girl and last week I went and got the step B swab again, when I went to my dr yesterday she told me the swab was negative!'!! Idk how that's possible I thought if you had it you have it kinda thing!!! I guess not!!! So she said I don't have to have antibiotics when I go to have this lil girl!!! I'm not complaining I just thought once u have it, u have it!!!!💖💖