Maybe she's really not ready for a serious relationship. Maybe she suddenly saw something in you that turned her off. Maybe she lost interest after the initial intensity started settling down. Maybe she is totally confused about what she wants. These things happen.
People can be unpredictable.
Perhaps it's better for this to happen after four months than four years. This will probably have the effect of making you very cautious about getting deeply involved quickly. That may be a good thing. Take it slowly, next time. I'd say it would be foolish to trust your heart to her again.