Bf + his mom + his ex issue

Hi! Okay so here’s what going on tell me what you think 💭

I’ve been with my bf for about 3 months and I’ve gotten to know his family and spent a lot of time with his parents. And his Mother always loves to tell me how much she hated his last gf and all This stuff. She tells me stuff about the ex and about the relationship her son & the ex had… she even tells me stuff that my bf has never even told me. Like about how the girl lived with them temporarily, and just all different things about the girl. And every time I end up feeling really upset. Because after that, I want to go to my bf for some explanations and stuff but it just kind of makes me feel like betrayed. I get emotional and upset about it. And I know he didn’t betray me, but it’s just the fact that I’m hearing it from someone else it makes me feel really weird and just not happy. And I just don’t understand why she tells me ? At first it felt kind of good because it made me feel good about myself being liked more than his ex but it very quickly started to get under my skin. A couple days ago she told me about how she used to take the ex and the ex’s little sister out to lunch and stuff… and then told me how the ex’s family used to come to their house for bonfires and I’m not sure what to do. Im not sure how I’m supposed to feel about all this… I wonder why she is even telling me any of this. Who would want to know all this? I know need to talk to my bf about this seriously, because I’m starting to feel doubts about being with him at all.

What would you do?