Is this a Magic Mouse? When was the last time you replaced the batteries?
Also, if it's a Magic Mouse, you may want to download and install Magic Prefs. It let's you customize your Magic Mouse or Magic Trackpad in all kinds of ways. But the best bit about it (other than the fact that it's free), is that you can tell it to display the Magic Prefs icon in the menu bar and tell that icon to reflect the available amount of battery you've got left.
Tom (... very nicely done piece of software.)