I agree with you. If my friend cares about being the first and has said it multiple times, I wouldn’t want to take that away from my friend. Or at least if it happens, I’d tell my friend earlier instead of telling them a month later. It’s a tricky situation that OP created, and the friend is a collateral damage of OP’s decision.

And maybe the friend isn’t even so selfish or entitled, maybe she’s reacting that way from being so embarrassed/hurt that she’s been going around telling people that she’s the first, but turns out OP took that away from her without even a simple heads up. It can even feel like her own best friend has betrayed her for doing that when OP already knows all these. Maybe OP’s friend is hurt that the OP hid the fact that they got married.

It can just be a simple clash of two different perspectives on getting married and different importance place on being the first to get married, but it’s also easy to see that OP’s friend is hurt.