That sounds like its all probably OK & that my concern of a rogue user having being created and your own user rights having been reduced was not the case.
A brief explanation of those parameters is at http://www.velvetblues.com/web-development-blog/how-to-add-an-admin-user-to-a-wordpress-database/ (I'm not suggesting you do anything with the tutorial, but it explains a little about the various parameters)
its good to be able to rule that out as the cause, but having never experienced anything similar I'm afraid I'm out of ideas at the moment for what else could cause this. I'll post again if I think of anything else to investigate to track this issue down.
Do you have a database backup taken soon before the issue appeared? If so is it possible to set up a new database and change the wp-config.php to link to that?