No, it is not transmitted sexually. It is extremely common and rarely causes problems. Here's some info about Strep B: https://www.mayoclinic.org/diseases-conditions/group-b-strep/symptoms-causes/syc-20351729
If the doctor believe it requires treatment, a simple round of common antibiotics will be prescribed. You did not and could not have given this to her.
Thanks Auntiejessie. I took 4th gen rapid combo test on the 41 day after the last protected exposure. The result was negative. Recently I experience jock itch at my inner thigh followed by painful ulcer on my tongue besides the above Strep B diagnosis for my partner. I'm now worried that the result is false negative and these are the symptoms of hiv or any other stds.