Your reaction is ok, it is proportionate. It is not an overreaction. It is an excellent signal to you to change something.
Your mind is simply now processing your inner reaction and feelings.
You have a smiling wonderful popular bestie in front of you - but there's a huge mismatch with this image because there's a new tight group bestie engages in and possibly influences directly - and you have been excluded from it. If you feel some actively dislike you - I believe it. I believe you. It might not be as significant as active dislike, but there's a sense of rejection or something about you that you feel they don't value. I believe you. You are right.
So your reaction is proportionate. It is good you are reacting because it's how you grow through this crap and beyond people like this.
She is not your bestie OP. It doesn't mean she's going to sabotage your life, but there is a vulnerability in you or about you that people might exploit. This situation and your reaction to it is a call to you to explore that. ( Been there done it, still doing it in my own life ffs 😆)
It is ok for people to have separate social groups but something about this dynamic feels off to me. It's like this lady and her group are not really your tribe.
Think about any hobby, activity, interest that you might really enjoy that would involve new, different people. Nothing related to being a mum. Nothing infiltrated by the other mums if possible. Build your sense of feeling socially worthy ( this dynamic and situation is incredibly rejecting to you and understandably so). If you cultivate things that have nothing to do with the other women, don't even tell this bestie of yours. Something about this dynamic feels to me like she could be someone who sabotages. I would absolutely not share too much with her, keep your vulnerability to yourself and don't let her see it. Re group and find your social confidence through anything not connected to her. You will gradually feel so much better.