I can tell you exactly why she did that. It's not about "twisting the knife" in the usual sense...her ultimate objective is not to hurt you. Her ultimate objective is to get a reaction from you!! She would be very gratified if you would fly into a tizzie -- ANY kind of tizzie -- to demonstrate how much you CARE that she is gone. She is looking for validation of her existence. You are not reacting at all, which says to her that you don't care at all one way or the other. THAT must feel like absolute crap to her.
She's spinning out of control for sure...just be glad you're not in the middle of it. Do NOT get sucked in or it may take weeks to get back out.
That's my $0.02.