Hello, Darkblaez:
Is this a form of Mirroring?
It could be.
If so (or not) why if I am suppose to be the ex?
The hallmark mirroring, chameleon-like, behavior of pwBPD happens typically to pull one into their sphere. From my understanding, pwBPD have a hard time with self-identity and mirror everyone around them. Maybe she doesn't want you to remain an ex. Maybe she's trying to flatter you in order to recycle you... .or at the very least, keep you in her orbit.
... .and if not, then what is it?
I cannot think of anything her recent FB behavior would be other than her trying to get your attention, thereby, keeping you in her orbit.
Once upon a time, there were two modes of communication: Verbal and nonverbal.
Now, there's Facebook.
-Speck