My "shy" girl

So Elizabeth isn't a shy girl...she only gets that way around people she doesn't know well.  Then once she decides you're ok, it's non-stop talking, and I do mean non-stop.  Last semester in Kindermusik, Elizabeth was non-stop talking, jumping, and everything else under the sun.  But lately, even though she has EVERY song memorized (which she sings to me all the time) and knows the book we read in it backwards & forwards, she won't speak.  I don't understand it.  She play with the other kids and even come tell me that they hugged her etc.  But if someone speaks to her, she instantly goes quiet.  Now I wouldn't be concerned as I always know she needs time to warm up to people, but we've been to a lot of classes and some of the moms/kids are ones we had last semester.  Even stranger, lately she doesn't want to talk in story time at the library.  That hasn't changed at all...I'm not sure what is going on, but part of me has to wonder if my "shy" girl is sensing that something is about to change.  Then again, maybe it's just a phase they hit when their two.