Strep B is transient which is why many Obs don't even bother testing for it... so you could be negative now and positive later or positive now and negative later...

My Ob said to me that he would only bother giving me antibiotics for it if I showed signs of a fever in labour or had a high risk baby, e.g. prem... and he was an infertility specialist so not one to take risks, but rather err on the side of caution. The risk of the baby actually inhaling the bacteria is very low. They can treat them after the birth if it does happen, but like I say, many take a wait and see approach. Antibiotics during labour as a preventative have both pros and cons to investigate.

ETA: Snap - posted same time as you feeb!