It means different things to different people, but I think it means that she really, really likes you.
Don't get too excited, it may not be 'like' romantically, but she's certainly feeling a lot of affection towards you. In calling you sweet, she is showing appreciation for you, probably for something you've said or done for her that meant a lot to her. She likes you! If you like her, you should keep on being your own sweet self, because this COULD go somewhere.
It's way more meaningful than just being physically attracted to someone. She finds you appealing. If she's attracted to you as well, she won't be able to resist you.
Good luck.