Strep B question??

Margaret 🇨🇺 • First time mother! Aliza Sarai 💖👶🏻 (October 31st 17`) 💍 to the love of life and best friends since 11•3•08 👨‍👩‍👧

I just got my results and they were negative but my question is how does one get strep b? I've heard if you have had anal sex before (which my husband and I have *sorry if TMI*), but it was still negative. Should I mention something to my midwife?