Plagiarism or just bad coincidence

Okay so today it was a pretty laid back day, until five minutes before I went to grab a bite to eat. My [Psychology]professor e-mailed me saying that there were a couple of things wrong with my homework, one of them was “plagiarism”. Now I understand how plagiarism is not cool. As a song writer one of my worst fears is that someone might take my songs and make them their own, even though they’re not very good, but you get the picture. Now for homework you can use the book right, so when I went to Wikipedia to get some more information about the subject, it’s automatically plagiarism? ummmm… okay…. ?_? what the hell? I wasn’t trying to get the answers off of someone else plus if you think that, that is plagiarism, then what is taking the answers from the textbook? If you ask me, that sounds a little hypocritical. Anyway, I apologized to him cause I like Psychology and I want to earn the credits now plus I might want to go into music, but I want my degree first, so I can’t afford to get expelled. He still hasn’t responded to me yet. So is it plagiarism or is it just a bad coincidence? by the way that’s just one question that going on through my mind, so you guys don’t have to answer that question.