Android : Does android app's signature change if I haven't changed the keystore?

I've created a keystore for my app and has always been signing with it. In the app there's a verification to test if the key hash has changed (means somebody else repackaged the app). However today when I exported the apk with eclipse and installed it, the key hash has changed.

Since I've never done anything to the keystore, I suppose i shouldn't change.

Go to here to find how I get the hash. http://javatechig.com/android/how-to-get-key-hashes-for-android-facebook-app

Thanks in Advance.