I said "Do you want to go out for dinner or drinks on Saturday night? I know [friend] wanted to do a double date soon"

She said "I'm sorry honey, I'm actually going out that night"

I asked her what she was doing and she said her cousin was having a party.

Regarding going to parties without me, she will have a girls' night here and there, naturally I don't go to those, but I honestly can't remember her ever going to an actual party without me since the first couple weeks of us dating.

It could very well be a cousin thing, in which case I feel I might come off as entitled if I ask why I wasn't invited. If that's the case, and her ex happens to be there because he's a childhood friend of her cousins, that's fine. If it's not just a cousin thing, and especially other people there are bringing their significant others, I'd be really hurt.