I [21M] need advice on how to communicate with an ex-partner [20F] about a boundary concern. How can I start this conversation smoothly?

We had been together for almost three years when we broke up last year. I used to get irritated easily and yell at her over small things. Because of that, she decided to end the relationship, telling me she was tired of me raising my voice. After that, we went completely no-contact for about a month. Eventually, we started talking again and hanging out occasionally. She gave me a chance to change, and I’ve been working hard on myself ever since. She really is the best woman in the world, and I love her deeply.

However, recently, while we were eating at home, I saw a notification pop up on her phone. Someone had messaged her calling her "baby." She immediately swiped the notification away. Right after that, she acted like nothing had happened until she went home.

I still haven't brought it up to her, and I'm feeling really conflicted on how to navigate this conversation without causing a step backward in our progress. How can I communicate my feelings about this message calmly and effectively?