UAC Switching User's Passwords?

I'm having here is that my user accounts have sort of switched themselves. To clarify, my Administrator account had a password, my other user account did not. I woke up to find that the Administrator account no longer had a password, and the user account was now passworded with the administrator's! Now I can assure you that nobody is playing a practical joke, because I am the ONLY person who knows the Administrator password, therefore there is no way anyone else could set that exact password for the user account.