I can see why it’s so upsetting, esp as you asked if anyone was doing anything at the weekend.
Personally, I wouldn’t cope with not knowing what had happened and iD have to bring it up. I’d either ask the person I was closest to and say I was aware of the night out and didn’t understand why I hadn’t been invited and ask if she could throw any light on it ….I’d put it as a genuine question rather than a rant.
The alternative would be to ask the Q on WhatsApp or however you communicate. However, I think the word with one friend is better. Using WhatsApp/text/social media means your tone gets lost and it could provoke a group thing where they feel guilty or go into cahoots to justify themselves etc and you do t really get an honest answer.
Id have the quiet word with a friend tomorrow. I’d hope they’d be honest. And after that, it will hopefully be clearer what next steps are. Possibly someone things happened OP that you’re unaware of and someone is offended by something they think you’ve done. Well if this is the issue, you can address it. It could be simply that the others do feel closer to each other than you. If this is the case, you can tell the friend that you’re surprised and quite hurt and either back off from them, or simply try to 7d rest and and tell friend that you’d really like to be more involved. Or it’s possible there was an oversight and they thought you’d been invited. These things can happen.
In the end, you probably need to know if this was a deliberate and pointed attempt to exclude you, a case of others being closer than you are (although you hadn’t realised it) or a genuine mistake. And then on the basis of that you can decide what to do next.
Throwing a stop probably won’t achieve anything. You can back off from the friends but as you say, this is your entire friendship group, so it would be a big step and not to be taken lightly. There may well be a way through, although quite what your role in the group is and how you feel about them all might need some adjusting. However, it probably doesn’t need to be as black and white as you’re friends or not friends with them.
I really hope this isn’t a deliberate and pointed attempt to exclude you. That sounds like something 13 year old girls do. Most adult women’s groups wouldn’t do that, but some would. And I guess OP you probably have a sense of if they would do that, or there’s something less nasty going on.
Anyway, hope you are able to get a bit of clarification and are able to move forwards with it for your own peace of mind. I’d try to retain an open mind to there being different possible explanations for it, rather than assuming the worst. By doing this, you keep the doors and your own mind open to moving forward with the friendships…and if they can continue, that would probably be the best outcome.