The same thing happened to me too. DS came home from preschool one day with a loose tooth. I was sure that he had knocked it loose at school (he mentioned bumping his mouth on something) and had him checked out by our dentist. As it turned out, it wasn't from the bump at all. The x-ray showed the next tooth ready and waiting to come in! His second loose tooth came a few months later. Oh, and he wasn't early getting his baby teeth - he followed the "schedule" for almost every tooth!
The best part - he had both front teeth missing at Christmas - so I have the cutest picture of him with the missing teeth and I sing that little song in my head every time I look at it..."All I want for Chtistmas is my two front teeth, my two front teeth, yes my two front teeth...!"