I was thinking that many people (including Christians) can have the impression that Christians aren't allowed to be angry. Or upset. Or just plain frustrated. We're supposed to endure everything quietly, just like Jesus endured the cross. But we forget that Jesus himself caused a revolution right before that, becoming very frustrated several times in the process. And he wasn't afraid to let people know!
I think there is a difference, though, in being rightfully frustrated, or being just a pain. Like I heard someone say, "don't be a porcupine - someone with a lot of very good points, but no one wants to be around them" :). Isn't it genious?? What good is it to be right, when you're so annoying that no one wants to listen to you anyway?
Like in Jonah's story - God told him to go tell some horrible people that if they didn't repent, they'd die. So they did, and then Jonah was all upset because he didn't want them to be forgiven. It was when God told him, "Have you any right to be angry?" (Jonah 4:4) In other words, "hey, you're not thinking straight here. I know you don't want them to get away with it, but this is people who can't tell their right hand from their left [not paraphrasing this part]. They don't know what they're doing, and I don't want them to die, as much as I don't want you to die. So nope, you don't get to throw a fit."
I think that God's "fit" is the kind that prompts positive change, and is for the good of everybody, not just for your need to vent. I don't think it's a sin to be frustrated, it's a human emotion, but what we do with it is what counts. It might be that we just didn't get our chocolate of the day, or it might be a sign that something bigger needs to change.