I would try these things:
1. restart your computer and see if the problem is fixed
2. Your time machine preference file may be corrupt.
1. hold down the option key
2. in the finder, in the Go menu, go to Library
3. in Library, open Preferences
4. find this file in Preferences and drag it to the trash:
5. restart and see if the problem is fixed
This will generate new system preference files with default settings so you will have to reenter your preferences.
3. create a new test user account and see if the problem persists.
Here is a general troubleshooting reference: Mac OS X: How to troubleshoot a software issue
this should be enough to get you started.