Husband and his female friend

Posting anonymously, because I would rather not have anyone I know find out. My husband has this female friend that he dated for one day, she broke up with him because of extreme jealousy. She was the person directly before me. Fast forward, we're happily married expecting our first she is happily married expecting her first. They kept in touch, even when my husband was overseas for almost a year she kept in touch with him. She still messages him & a while ago she messaged him when we were on a date, a rare occurance when life is busy with a toddler. It was just a simple date at Steak n Shake, but he responded to her text during the middle of our date... I never had a good vibe/ feeling about her. Anyway, I saw her text & it came across as flirty, she even called him a cutesy nickname & I did not like it. I have told him how jealous she makes me & it's weird that she messages him every once in a while like once every few months. Today she messaged him again while he was putting together his father's day gift but this time she was saying something along the lines of "so you went MIA on me." Well he deleted the messages this time... Even though he told me she messaged him, I still think it's at least a little weird. I was telling him again how much I don't like her this evening & how it bothers me that he keeps talking to her knowing how much it bothers me. Then he said they would probably talk more if it didn't bother me as much. In the back of my head I'm starting to think it's more flirty or possibly emotionally cheating or at least the beginning of emotionally cheating. What else could I do in this situation? I'm starting to consider asking her to not message my husband? Am I overreacting?