Every woman has group B Strep (it's part of the natural microflora that inhabits your vagina), but they are actually testing for levels of the bacteria. When you test positive, it means you have a higher than optimal bacteria count. It's not a problem at all when treated with antibiotics, which will bring the count to a normal level. It typically doesn't even cause problems when untreated, but it can. It's more of a precaution than anything.