This has happened several times, I login to find myself at a table or a game that I didn't join, changed my password and it still happens...??
That's really strange. Are you in an active game, or on the end-of-game screen? The latter I've seen happen to me on occasion when I closed the browser after game without logging out. On the next login, I was at the end of this game, usually with opponent's name changes to some number.
Are you closing the tabs on games before they finish? That could explain it.