Melissa
She was probably hoping that you would have acted on her interest in you sooner. She was sending some signals your way through smiling at you but she may have eventually gotten frustrated when you wouldn’t act on it. By the time you started smiling back at her, she may have already lost her patience and just decided to start ignoring you. I have been in this position before. A guy I really liked wouldn’t act on all the signals I was sending him until I got tired of it and just started ignoring him. He started trying to be nice to me later on and have small talk but I had already moved on.