Well, if you were checking your Gmail account over some open wireless somewhere... it isn't too hard for someone to have gotten in to send mail out and whatnot. Not sure if they could change your password without first entering your password though, not sure how Gmail is set up.. never used it. Like geekyone said though, contact Google and they should be able to fix it all for you. If you feel the account was jacked, I would suggest checking your computer you use for keyloggers.. and not using public computers to access your email.. or anything you put a password into.
"Bad.. Good? I'm the guy with the gun"