@lilyflower7,
You might have - it kind of depends on the circumstances. For example if it wasn't set in stone where they were going to meet - maybe that is why she didn't text you until the last minute. Maybe she just assumed that everyone would think directly in front made sense? Is the metro very close to the park so it isn't really an inconvenience? Or maybe a last minute change of plans for some reason - did you ask her why she didn't let you know earlier?
(this is just from what I can read here so I am making many assumptions) My take - she probably isn't the best organizer and probably flies by the seat of her pants - perhaps that is why you tried to nail her down to a specific spot knowing the way she is. She may have been trying to get a consensus from the other party goers or maybe as she was coming out of the metro she bumped into many of the invitees so it just made sense to text the few others to just meet there.
There could be many reasons why the last minute text - but if this was an informal get together and the change in location wasn't such that it was a hassle --- you had to drive or it took 20 minutes to walk from your previous location - what is really the big deal? They weren't going to leave without you if you were 5 minutes later?
I know she should have let you know earlier and you were trying to be organized, but if she is the type that is not organized like you, you kind of need to realize that and don't take it personally.
Now if it caused you the cost of a cab ride or another trip on the metro I can understand the frustration, but if it was just a walk around the corner, then I think you might have overreacted.