Jealousy over ex

Okay so me and my ex boyfriend broke up 2 months ago and during the relationship there was a girl named Grace and I went through their messages and there was nothing bad but her flirting but he told her to stop doing that and to not worry about her which was good and they stopped talking for like 6 months but now that me and him are over they've started talking again and all I see in their Instagram comments is him commenting like a flirty "gay af" and she'll be like "learnt it from you" and just like that flirty commenting right? It makes me so jealous and I get annoyed by it. I do not want to date my ex again or try and start over or anything like that, but I still get jealous?? Can someone pls tell me why that might be considering I don't want to date him every again?? thankyou ):