Restate tour iPod touch. Your iPod touch is a little tiny computer and they need to be restarted occasionally. Hold the home button and the sleep button down at the same time, till the unit shuts off then starts back up with the apple logo. That should fix anything! Also, don't forget to check for updates regularly.
It's usually because the device's memory is full. Jailbroken devices can clear memory with sbsettings and retail devices can be rebooted or there are apps that can clear the memory (look for "free memory")