Like “oh! I would only be able to go the original week of [dates] when I have the time off. I hope you’ll be able to move it back to that week so I can still go! I understand if you’ll have to go on the trip by yourself/without me. But if you can move it back and we could both go, I would really enjoy going with you ”
^ that approach may be more relevant if you think it is an intentional power play where her jerking you around or causing drama is somehow gratifying to her. On the other hand, If she is kind and understanding but simply doesn’t understand how time off works for many jobs, AND you genuinely want to go, it may be safe to reveal how worried you are.