Is it weird that my boyfriend is going to a girls birthday dinner party without me?

Anonymous
My boyfriend told me he was going to his girl friends birthday dinner party and didn’t invite me. He didn’t tell me until the day of and said “but there’s going to be a group of people so don’t worry”. At first I wasn’t bothered by it and didn’t think anything of it, but two different people who’ve asked what we’re doing tonight have said it’s weird he didn’t include me? I haven’t met this girl (hope she has a great birthday!) but usually plus ones are included, right? Like I’ve always assumed my friends being their people. I don't know what to think. If it is strange, should I say something? If so, what do I even say? I asked who she was and he said it’s some lifelong friend who is moving back to our town and made the point to tell me she’s recently single because her fiancé cheated on her? I don't know if that’s relevant information. Anyway.. should I be weirded out by not being included and if so, should I tell him? Lol
Is it weird that my boyfriend is going to a girls birthday dinner party without me?
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