Don’t wait. See a speech therapist NOW and one who truly has experience working with kids who stutter. But there is no way you caused this. Speech therapist isn’t just for her, but for you. They can help you to understand how to respond, and not respond. Consider seeking out family support groups. They can be excellent for all of you. If you are in the US, look into both friendswhostutter.org and westutter.org - both have an annual conference and links to resources. Friends does one day conferences throughout the US, and links for local chapters to join. I’m not well versed on resources outside the US, but they might have links for those resources as well.