Am I Overreacting because “Best friend” adds my boyfriends ex on IG after unexpectedly becoming co workers?

She texted me when she found out that my boyfriends ex was at her job site, turns out she’ll be working there as well she saw her and introduced herself. I replied to wow that’s crazy! and to also not mention anything about me or my boyfriend. (To avoid awkwardness?) she later texts me that they now follow each other on ig. She claims the reason they did was because the girl asked her what school she went to (the ex went to the same school but left before my best friend started) and she said omg I went there too add me on ig! She felt like she couldn’t say no because it’ll look sus. She could’ve simply said I don’t use IG like that. I thought this was weird to say because she removed every single person from that school years later after graduating because she ended up being in a relationship with her English teacher. So I feel like in a way she wanted to have something in common or to talk about. Which makes me believe she was being more than friendly with her instead of work friendly because the girl felt bold enough to ask her for her ig that quick. She could’ve mentioned another school she went to before that one. I just want to put it out there that my issue isn’t towards the ex but more so my “best friend” and how she handled it. A little insight about her to explain my frustration: she has a history of being a compulsive liar so I can’t help but think that she’s lying to me and does not care about my feelings. Recently her friends stop being friends with her because they were tired of how selfish and self centered she was, and felt weird/hurt that they’re still friends with me. I tried my best to fix their friendships but they simply were tired of her. She mentioned this when we were arguing about the situation about IG and the ex. Now I feel like she has that against me and it is the reason she handled the situation this way.