Demons shake me in by bed sometimes. Relax. It happens to more people than you think. People just don't talk about it. First, they want to freak you out. Eventually you will stop freaking out and they won't have as much fun or power over you. Also, you need to learn to start scaring them. Remind them that Jesus is Lord. They hate the name of Jesus. Remember, that if you are a committed follower of Christ, that God is on your side, and this demon is a weak enemy of God. Look to God. Depend on him. Tap into his wisdom and power to fight this demon. Don't be afraid of them. Again, refuse to give into fear. Eventually, they will become nuisances rather than terrors. FAITH. Faith is huge. Don't let them make you believe that they are in contro. Don't let them make you believe that God doesn't have victory in store for you. Don't let them make you believe things that aren't true. The truth is your shield and buckler. Read the Bible. Believe in the Bible, and don't let them act like they know better than the Bible.
As far as why they are doing it, who knows. That is like asking why people are mean to people. Demons are bad and they like causing trouble. I wouldn't ask why so much as I would talk to God about the situation. I like to see situations like this as an opportunity to gain understanding of the spiritual realm and to draw closer to God.
I would look up Bible verses with the word devil, evil sprit, demon, etc, and find out more. There is a whole world of learning and training that you need here. That demon is just scaring you. It is not a very strong demon. Later, you will need to learn how to battle stronger ones.
Walk closer to Christ. Learn that, and you will learn not only how to fight demons, but many things you need to know.
email me if you have more questions.