it’s just I don’t get it and I’m wracking my brain to try to understand why would she do that since she left me to go back to her ex.
it's not logical. pwBPD (people with BPD) don't process information and emotions quite the same way you or I would. so what would make sense for you wouldn't for her and vice versa.
people who are on the BPD spectrum have harmfully intense chaotic emotions that switch rapidly. they also believe those emotions are caused by external events, people, places things.
sometimes with those intense chaotic emotions they
unconsciously look to make their external world match their internal world. make sense? its not a deliberate choice, it's more a maladaptive coping mechanism.
something stirred her up, maybe it was her current partner, maybe it was a memory, maybe something else entirely. and to rid herself of the emotion she lashed out. sharing the emotion makes it less intense for her.
we could speculate forever on what made her post. and never really know for sure. maybe she wanted attention,... so she puts up a provocative post. maybe she had a moment of missing you, or thinking about you and felt she had to devalue you just as you had rejected her friendship.
my experience was that much of what my Ex did and said was about handling her intense emotions.