Gracie, I was found to be Strep B positive with my first pregnancy and have been treated as if positive ever since. The other ladies are spot on with their info as to how and why you are given the antibiotics (which are perfectly safe).
Just be aware that if your waters break before you go into labour you should go to hospital. Ring them and let them know what has happened and tell them you are strep B positive. Make sure you stress that you are strep B positive. Most hospitals don't like you to hang around once your waters break if you are positive. If labour doesn't start pretty soon after the breakage they like to get you started so they can introduce the antibiotics. There is nothing to worry about. This happened with my daughter Grace.

All the best,
Debbie