AITAH for responding to my exes text while in a relationship

So my ex texted me last night. It was a ldr we were on and off for about a year and a half. Obviously things ended and I haven’t talked to her since last year August. Until I saw I text where she said she knows it’s random but she hopes I’m so happy. I appreciated that after everything we’ve been through but I know that’s the past and I’m more than okay with it as I have a wonderful girlfriend now. Nostalgia hit me and I updated her that I had a girlfriend and that life was okay, she proceeded to say she has a man she wants to marry in the future. Here’s where I believe sympathy for me may dissipate. Few days ago me and my girl gave boundaries to eachother that there should be no contact with exes and I agreed. Solely cause I never thought I’d be in contact with that ex ever again and she was my only ex so I didn’t feel like there needed to be much discourse around it.

I know I kept the conversation with me and my ex as surface level as I can be because I wanted to respect my relationship but also to be true to myself, but it has come to the cost of my girlfriend not feeling secure.

Is it a horrible thing that I did respond? AITAH for being open to a cordial update with someone from my past?