My son goes through a stuttering phase at least twice a year, so far. Every time he makes a cognitive leap it takes awhile for either his vocabulary, or the muscles in his mouth, to catch up. :)
Of course, if it's not just that your dd is running ahead of herself, speech pathologists are amazing (okay, I'm biased, I have two in my extended family). To ease your mind, look some up online and read about what they do. Education is relaxation...at least in that if it IS a problem it's not life threatening AND totally fixable. Even for free...since school districts keep speech pathologists on staff. :)
My mother gives me a hard time every time my son starts stuttering though. It's such a definite pattern with him I've learned to ignore her. But I DID talk to my god-cousin the speech pathologist...so I'm armed not only with my own observations but with the "Even Chris agrees with me that it's a totally normal process. She say that not all kids go through it, but most bright kids do." Hehehe...take that Nana. Not only an advanced degree, but the intimation that your grandson isn't a turnip. Somehow though, it has to be brought up every time. Well, that's moms for you.
Z