It's around the time where she makes the decision if this is something that is going to work long term. If not then they usually cut bait because they don't want to become emotionally involved past that point.
It's hard to always know why it is, or what she was looking for, but in some way, you weren't it.
No reason to beat yourself up over it, it happens. Maybe she wanted more commitment from you, more security from you, more lifestyle from you, etc...she might have mentioned something in passing a few times, which is how they usually bring it up, but sometimes not...either way it's better to simply shrug your shoulders and find someone else.