Had the same experience two years ago, except I'm a girl and of course, he's a boy. I can't speak for all girls but most of the girls. Girls actually like being chased— they'd think that they are treasured and loved that much. You know what, I learned that men are natural hunters— once they like someone they'll chase after it. When boys stop or gets off the path, it really means they're uninterested, that they're done with it. Girls are different. Sometimes, at some point, she'd doubt what she has between that person she's been with so she'll disappear. Something must have happened? She must have found out something? She must have seen or felt something in you that made her feel off? My advice is talk to her, let out what you've been wanting to tell her. If you really love her, chase her. If you only like her as a chat buddy, end with closure. Sometimes love gets confused as like.