Could be many things. The most common is that internal storage has been entirely filled. You should try to keep around 1GB free as it appears it is used for swap space.
Alternatively, you may have installed a poorly coded app that is eating up CPU. Either in apps - or if that doesn't help, battery stats - see if one app appears to be using an inordinate amount of resource. If it is, try disabling it to see if that helps.
If none of this helps, try re-posting with a little more information.
