I don't know the answer about how to tell. I don't have BPD, but from living with it (BF has, so my experience is from the other side), can say that it is as if every good thing ever suddenly flies out the window like it never happened, when an expectation (stated or unstated) is not met. Black and white thinking doesn't lend itself to good decision-making.

So I can see how it would be difficult to tell. Hopefully others will have some advice on that.

BeyondtheRainbow covered a lot to which I'll just say ditto.

Your quote is above, because it caught my eye when you said you feel it's most likely because you're mad at her. You are probably right on that.

The meaning of "bursts your bubble" -- yes, I strongly believe that's what she meant. Interactions. Life. It's the nature of the BPD beast.

The being late/no shows are troubling. That is a legit negative point, and the others are right. That calls for a calm discussion (write it out ahead if that helps). Boundaries are a big issue here, especially around availability.

So, long/short, to your title question: yes, I think you are over-reacting (due to unrealistic availability expectations). But I also think that she over-extended in offering unrealistic availability. It really does call for a solid conversation.