Yes, this happens with me too. I go around telling people that I am an introvert because I usually don't start conversation and talk only when question is directed to me but I have been told by my friends you are an extrovert and talk a lot when I become comfortable with them. But when I am in larger groups I keep shut so people don't notice that I stammer and I can shield it. This stammering has affected by life in so many levels especially dating. I think my whole personality has changed because of this, I have molded myself.