First, be sure you know that what you are assuming is correct. You having Group B strep does not mean you got it from your fiance, Group B can exist in both men's and women's digestive tracts and intestines, and sometimes it pops up and other times it does not. You could have had it for a long time, and also, you could give it to her and not the other way around. She should get tested for it.
You also are assuming that it will be gone once you take the antibiotics, but antibiotics don't thoroughly get rid of it in people who are considered to be "colonized." Those people sometimes test negative and sometimes test positive. (And, in fact, giving antibiotics right before a woman gives birth in order to protect the baby is also not enough to permanently remove Group B strep from someone who has it in her intestines, it just dampens the possibility for the weeks around birth.)
Group B strep isn't considered an STD, it's a kind of bacteria that lives in the body. It's said 25% of women have it, I don't know the numbers for men.
So, my suggestion is, ask your fiance to get tested, without blaming her for being the source. (Even if she tests positive, don't blame her for being the source, since you could have been the source for her.) If she tests positive, she should discuss with her doctor the possibility that antibiotics seem not to be totally effective at addressing Group B in a person who is 'colonized,' and ask for advice as to whether there is any point in taking antibiotics now. (Doctors test women for Group B before childbirth, in order to give the woman antibiotics if she has it.)
If I were her and you called to say you have Group B strep, I'd get tested. If I find I don't have it, I'd be asking the doctor how to keep myself from getting it from you.
Here is a link to a good article on the subject. Its main thrust is preventing Group B strep in babies, so it goes that direction, but its FAQs are good in any case.
From that article:
"Like many bacteria, Group B Strep may be passed from one person to another through skin to skin contact, for example, hand contact [...] GBS bacteria do not cause symptoms or discomfort. GBS carriage is not a sexually transmitted disease, nor is GBS carriage a sign of ill health or poor hygiene. No one should ever feel guilty or dirty for carrying GBS – it’s normal.
"Because GBS may be passed from one person to another by skin to skin contact, everyone (regardless of whether they know they carry GBS) should wash their hands properly and dry them properly before handling a newborn baby."
Again, I'd encourage you not to jump to the conclusion that the sexual activity with your fiance gave you Group B strep without more proof. Apparently there are good tests as well as less good ones, (the article names the tests). If your fiance gets one of the better tests, it can find out reliably whether or not she has Group B strep.