Dealing with Rage in a Borderline Relationship

I’m in my first long-term and serious relationship and it’s going well. However, as a borderline there are some thing i’ve noticed and idk why I feel this way… just like I do he has a toxic ex and for some reason when we talk about our relationship past hearing him talk about their sex life fills me with rage. It’s not the fact he’s talking about it because he’s blatantly expressing the traumatizing experience he’s had but it’s the thought of him involved sexually with this man that enrages me. What the fuck is this feeling and why is it grabbing me now, his ex is long gone and nowhere near here so that will never be an issue but wtf