My baby is 3 weeks old, too and she does the exact same thing.... as all 4 of mine did that as babies, I'm fairly certain it's normal infant behavior. I wouldn't give the GBS thing a second thought. The antibiotics would've taken care of that. I was GBS positive in at least 2 of my 4 pregnancies (possibly 3, but I can't remember for sure), and my kids are all healthy and perfect.
Congrats on your new little one!