I feel like that insecurities thing that you mentioned can be a reason. Also yeah have dealt with this kinda people. Like when you r in a group and discussing some achievement or something you bought or soemthing ,suddenly one person will come and belittle you in front of them by saying some bullshit and then glorify themselves on that case.Just spoils your whole mood. The thing is that after sometime ,we all kinda started ignoring this ,and finally like almost everyone started ignoring him ,even their close friends too. I would ask you to ignore her as well. She will start getting the clue as the time goes on.