-Check your system mouse preferences (It could be possible that due some bug or whatever, they got mixed up).
-Check your mouse for dirt; open it up and give it a good clean.
-Restart computer (It could be possible that some program *beeped* up)
-Check mouse cables (It could be possible that the cables got damaged)
-Check mouse plug (It could be possible that the plug got pulled a little and thus giving bad contact)
-Check mouse batteries if you have a wireless/optical mouse
-...
I used to have this problem til my mouse broke. It could either be poor contact or driver conflicts. I suggest searching for the correct driver for your mouse that uses all of the extra buttons to its advantage and installing it.