A lot of people started calling me "K" in my late teens and beyond. I guess "Keirsten" is just too long to say all the time. My brother used to call me "Keirsty" when I was little, and soon my mom and dad picked up on it too. He started it the first time he saw me after I was born....he walked right up to me and said, "Hi Keirsty."
My mom also used to call me "Tootsie." That stuck for a while too.