Am I overreacting for being mad that my best friend didn’t invite me to her birthday party, but invited everyone else?

I (19F) have been best friends with “Emily” (20F) for years. We do almost everything together, so I assumed I’d be at her birthday party. Then, I saw a bunch of our mutual friends posting stories from the party… and I wasn’t invited.

I asked Emily about it, and she just said, “Oh, it was a last-minute thing, I didn’t think you’d mind.” But clearly, she thought to invite everyone else??

I feel really hurt and left out, but I don’t want to seem clingy or dramatic. Am I overreacting for being upset about this?