This was not a relationship. This was a little fling. Don't give it more importance than that.
You say it was not a rebound but guess again - it WAS a rebound for you. Your first Post D dating experience. And you really really really wanted it to work. But it didn't. For whatever reason. My thought is that you came on WAY too strong, WAY too fast.
But the good news is - you know you can have those loving feelings again (and I'm sure that's what it was not real love so soon). You will have it again - just don't give out your love so easily or so soon. It was not meant to be because it takes 2 people to want it and when one wants out - that's it - that's the end.
You can learn a lot from this experience. You will find someone else and I will bet you will be a bit more cautious. But that's good. Maybe she didn't want you because she knew she had you. People tend to want the things they have to work a bit harder for.
Now back off and stop messaging her. When one person says it's over and the other shows up at their work with origami cats - it is obsessive and could be considered stalking. Don't let that happen.
You may never know WHY. But it really doesn't change things to find out. Barb