Hi limetaste,
I am not an expert, and still learning about BPD tendencies and traits, but I can say that I observed my ex yo-yoing between former acquaintances and friends who for one reason or another had fallen out of her good graces. Some had done nasty stuff to her in the past, others she'd pushed away because of some perceived and exaggerated slight. There was often no rhyme or reason for the timing that I could see, but pwBPD seem to thrive on drama. Also, given your recent break-up, she was likely feeling scared and lonely and vulnerable (pwBPD can't stand to be alone), and may have reached out to the first person she thought of, someone she thought might be sympathetic or fill some need of hers regardless of what this person might have done to her in the past. When they white-paint, they see none of the flaws. When they black-paint, flaws are all they see.
I know it's hard, but for your own well-being I would suggest that you try to resist the urge to follow your ex's social media, and stop keeping tabs on her. Otherwise you'll end up getting sucked back into the drama, and you'll be the one who ends up getting hurt. My ex and I have a child together, so walking away completely isn't an option for us. But it sounds like it could be for you.
Thank you for your answer. You are right, it's destructive to keep tabs on her, but it's a way for me to find a closure. I know getting back with her can never be a reality, even if she contact me. The same cycle will repeat itself until she gets help. By checking her social medias from time to time I'm seeing more and more signs that she's a BPD. In some way this helps me moving on, understanding more and more how she works and why she ghosted me etc.
I also have a child with another BPD from a "former life" many years ago. One could think I should've seen the warning signs in this new relationship, but I guess all BPD's are different in many ways.