Question on Boundaries

I was listening to the Podcast Episode 163: Boundaries 2.0 and Brooke mentioned that when somebody asks her, “Is this a boundary violation? Do I need to set a boundary”, 9 out of 10, her answer is No.
So, my Ex-Husband keeps asking me about my new partner and our relationship. Where we stand, what our plans are. He says he wants to understand it because it’s not easy for him to accept I have a new partner. He is in a new relationship as well.
I try to explain to him that my relationship is my business and I will not discuss it with him. He keeps going on and on. Doesn’t stop asking. I find myself justifying. I get very angry. I feel my chest tightening and under pressure. He wants explanations and doesn’t let go. This has been going on for over 3 weeks now.
I’m not sure if this is a boundary violation. Or if I just need to work on my thoughts to be able to say No calmly without justifying or explaining my relationship. We have kids, so we are in regular contact.
Thank you for your help!