hi sweet tooth,
How are you doing today with the silent treatment? Did you know that the Silent treatment is considered a form of verbal/emotional abuse? Really. It is. You can read more at this link
BPD BEHAVIORS: Silent treatment - verbal abuse.That's why it's driving you crazy. because emotional abuse takes the joy and vitality out of life through the distortions of reality, because the abuser’s response does not coincide with the sender’s communication.
I decided to set a boundary about it if/when she contacts me again. I'm going to tell her she needs to tell me when she feels overwhelmed rather than just blow me off.
boundaries are good things. I am all in favor of boundaries

they are great stuff for us.
Have you thought about how you are going to communicate this to her? Have you practiced a SET? Or a DEARMAN? I am going with the idea that you want the best possible chance of success. If so it would be a good idea to put some thought into how to deliver your message.
Which does she respond to better? SET or Validation?
'ducks