Mac screen capture not working?

A cople of minutes ago, I took a screen capture on my mac using command+shift+F4. It worked fine. However, I needed to take another screen capture, but when I pressed command+shift+F4, it didn't do anything. I tried pressing command+shift+F3 (whole screen capture), and instead of taking a picture, all my programs flew to the side of the screen. Have I done anything? How can I get screen capture working again?