Why would she disappear so quickly? Toxic shame? Triggering her? I just don't get it.
It could be that she was just testing the waters with POF. A lot of people who are thinking about using dating services go online, look around, try to decide if they are in the mood to do it. Could be she just broke up with someone or hasn't dated in a while and is getting ready to put herself out there.
It could also be that she saw you and felt uncomfortable with you being able to see into her life - pwBPD often over emote - she could feel violated - especially if something has happened in her life and she is feeling raw from it.
It's 18 months. A lot has transpired.
Your's was a 6 month deal. The breakup hit you hard. You've been working on yourself. What have been your greatest revelations?