I had strep b with one pregnancy, and have been treated for it with three pregnancies. When it was first discovered, they treated me first with a ten day course of antibiotics. When I went into labor the treated me prophylactically with I.V. antibiotics. As I understand it, even if it is left untreated, there is only a slight risk to the baby but why take the risk. As well, my OB said that the infection could develop between treating and delivery, so he always administers prophylactically in woman who have been found to have the virus, just in case. In my two subsequent pregnancies, we just decided to treat with antibiotics at delivery (again, just in case).
So I have had three healthy deliveries following I.V. antibiotic adminstration during delivery, so don't worry. It is no big deal. Don't worry about it! Best of luck to you.