DDD, you're definitely not the only one who likes to know ahead of time. And you're not being childish by reacting badly to the sudden change in plans.
I am very very bad at spontaneity, and just don't cope well at all. Last night, a group of us took husband's best childhood friend R out for a birthday dinner. R's wife L and I get along very well, and as the only women in the group, were going to sit next to each other. As I stood behind my chair, about to sit down, difficult child (who is VERY bad at reading social situations) plopped himself down in it. It was all I could do not to pitch a hissy fit, because I had to reorganize in my head at the last minute how I was going to handle conversations at dinner now that I wasn't sitting with L. I adjusted, but the first 5 minutes of the dinner were not pretty.
I'm sure that you had all sorts of plans for how you were going to spend your week, which now need to be adjusted. It's not unreasonable at all to feel a bit put out by the change.
Trinity