Need advice again on how to respond to my W. I feel that she's trying to bait me to react.

It's going to be her birthday tomorrow and all of a sudden she dropped me a message saying that she won't be returning home tonight. She has stayovers previously in the weekends and this is the first time she's blatantly doing it on a work day. Ok, given that the next day is her birthday she might have plan something out with other people in her life.

When i saw that message, my heart hurts and I barely managed to stop myself from spewing all my words onto the phone. I wanted to messaged back "Why are you doing all these? Are you trying to prove a point? What will the kids think about you not being at home? You want to spend your time with your OM? Be my guest... yada yada yada..."

Or should i just replied with a short "OK." Will that do? Will that be considered being aloof?

I was planning to get her a card and a cake chosen by the kids tomorrow. Now i'm having second thoughts about this...

Some help please.

M(36), W(36), D(6), D(4)
M-7, T-8
Bomb Drop - Nov 28, 2019
W requested separation - Sep 30, 2020