It could be because you have the 'portrait orientation lock' on. This keeps your screen in portrait mode no matter what you do to it. To turn it off, double click the 'home' button, and a bar of four apps should show up at the bottom of your screen. In the bar, scroll to the left once and you will see your iPod controls. The app to the left of the iPod control is your portrait orientation lock. Touch it to turn it off and on.