Snufkin is right, it can come and go. 1 in 4 women is strep b+ at delivery (according to the leaflet I had from my hospital anyway!). It isn't routinely tested for in the UK because it is so common but as you've had it before they'll probably test you. 36 weeks is the time it is usually done because if you have it then it isn't likely to go by your edd.
I was strep b+ with my 2nd LO and had to have antibiotics through IV (unless you decline). They like you to have one 4 hour dose as a minimum (at my hospital it was 30 minutes of medicine through an IV, a break and then a 2 minute fast pump through the IV, I'm not sure if it is the same everywhere). If you don't get the minimum dose they will monitor LO, for us it was 12 hour observations but I've heard people say their LO was monitored for 24 hours.