Am I overreacting?!!!!

I have been with my boyfriend for 6 months now and we were friends long before that. He's always been really flirty with girls but it doesn't bother me. i suppose that's hard to explain. Anyway, there is this girl that we both have known for years and who is about 13 years older than him. He's always told me that he had a crush on her when he was like 15 years old and would flirt with her all the time. She got married about 4 years ago. Last year she began messaging him a lot before we started dating and would just flirt with him unashamedly. We found that really weird. Eventually he flat out asked her what her deal was and if she was trying to sleep with him. She said yes. He talked to her and told her that's not what he wanted, she was married and had kids, etc. She left him alone but would still message him from time to time. He hasn't really talked to her for the last several months and has even started ignoring her in person. Today he showed me a message from her from a few days ago in which she said "I know we haven't talked much anymore but I just wanted to tell you that you looked really nice today. I think about you all of the time." It was a long message along those lines. When he showed it to me, I felt nauseous. It makes me so uncomfortable. He kept telling me that he didn't do anything and that she just messaged him randomly. Eventually, he got upset that I was "making a big deal" over it. The thing is, I don't know if I am. I don't want her messaging him and he's told her that he doesn't want anything with her. Why won't she stop? Am I overreacting?