Password problems

After being gone for couple of weeks I attempted to log onto my account on my computer and it said that my password had changed even though i never changed it. So I logged into the Administrator account and tried to change it that way. I changed it and it sent me a message (something that i neglected to read) about having my old password not change or something like that. And now, I am in my account and everytime i try to change my password, the comp tells me to type in the old password that i don't know in. I was wondering if there was a default password that the computer automatically adds on or if there was another solution to this problem. Thanks in advance.

This ought to work: http://www.mac-forums.com/forums/showthread.php?t=61798&highlight=resetting+password