Hi Katykaty! Welcome to the family!
This is a good place to come. BPD is an often-misunderstood condition, almost impossible to truly understand for people who haven't experienced it. We "get it."
To answer your question, maybe. If we knew more about this other woman and the nature of both relationships, it may be easier to pick apart. For pwBPD, fear of abandonment is extremely strong and that could be playing into it here.
Has your ex been diagnosed? Not that that fully matters. Even if he hasn't been, you may still find the information on this site helpful.
When you're feeling up to it, I hope you'll look around at the tools on this site (in the drop-down tabs at the top), read (and post) other threads. And I hope you'll share more about your relationship. The more we know, the more we can help you try to understand.
Again, welcome!