Totally normal. And isn't it the cutest thing?? Well, with my daughter it was super cute anyway because she always had this huge smile on her face while she was shaking her head "no." My twin sons did it, too, as well as my 4 year old daughter when she was around 6 months or so. It's just a new sensation they get when they do it. It messes with their balance and equilibrium a little and they can get a little dizzy, so it's fun for them. It's not dangerous or anything, she's just exploring her body and her abilities. Learning to control her head. Strengthening her neck muscles. Don't worry about it too much. If it seems she's hurting herself or something, then check it out with a doctor, but most likely, it's just normal.