I've found little girls to be very funny creatures. And, yes, I suppose all of us mommy's were little girls once, too, lol!
I don't think your daughter knows why she suddenly feels so cranky. Most likely, it's the let down that follows the excitement. My daughter does exactly the same thing, as if nothing good ever lives up to expectations. I think what happens is that in her imagination everything is so perfect, and it is going to make her so very happy, that reality can never live up. The dilemma of the perfectionist (which many little girls, like my daughter, seem to naturally be). I really don't think "spoiled" is the right word, although my daughter can be spoiled about some things, but emotionally it seems to come from a different place, not from a sense of entitlement.
What I'm learning to do with my daughter is simply tone everything down. Lower expectations going in, keep things very simple, treat less often, etc. Experience the joy of sitting in our PJ's reading books together, or making a puzzle, and then talking about how nice that was. Learning that these are the best "treats." Ultimately, my daughter really is a simple child, who likes to stay close to home, not have much change, watch a lot of movies, talk with her friends, and shop (not that she needs a thing - we talk a lot about that, too, about choices AND what the heck you DO with all that stuff!).
The funny thing is that material things can't make our children happy, but the children believe they can. And, often, us parents believe the same (we are, after, products of the same society our children are, and the subliminal messages of advertising are a tough thing to fight). It's like having been sold a false bill of goods. The more we can do to help them reconcile the reality of the experience with the false expectation, the happier the child - and the parent - will be. They need to learn, for example, that creating a new room is fun, and something they will subtly appreciate for a long time, but isn't a magic bullet that will suddenly make every day glorious. And, yes, the problem is that she believed it would.