That might not be the nervousness really -- more along the lines of "should I kiss him, is he going to kiss me, did those onions I had for dinner give me bad breath, is this dress too revealing, is it too boring," etc. etc. etc.
That's all guesses though of course but it's not that surprising that she'd be nervous when you're only two weeks into a dating relationship, especially if she started it all (i.e. she has strong enough feelings for you that she took a chance and let you know).
Then of course if you've been friends for three years that's probably another level of nervousness (she really wouldn't want to ruin the friendship if the romantic relationship doesn't work out).