A girl who used to like me has a boyfriend now. Now, she is acting really bitchy and mean to me (and others too, but to me she is especially poignant). I genuinely do not care, but I want to understand the psychology behind it.

Just simple stuff. Used to like me, wanted me to escalate, but I did not cause I was not interested. However, I did flirt a bit so I kind of kept her hanging. There is this (evidently lesser SMV) dude that she also liked before. They got closer and now they are in a relationship. After that happened, she has become super bitchy to me (and other people too, but as I said she is really poignant to me). One of her friends (who also wanted something but I shut her down) is doing the same thing too. I genuinely do not care about either of them as I am still not interested. However, I want to know why women would do something like this. Just to build my knowledge base. I am guessing because I "rejected" them and now they are in a more secure position, they are using their safer situation to hurt me as much as I "hurt" them?