Hhmm tricky one.. If you were using the shortcut keys I would have said maybe you've accidentally turned on sticky keys or something like that but if you are using the Copy & Paste buttons on the Ribbon then sticky keys wouldn't be affected.
Are you able to take a screen shot (just press the PrtScn button on your keyboard) of what the screen looks like when the signature menu appears and open a blank document in Word and press the Paste button and it should insert a screen shot and upload here?