I think Louise is on the right track. There is a username that you use to log in, and a friendly name that appears alongside your posts and comments.
If you go to your profile page and edit your personal information, then you will be able to change your friendly name to anything you like, including your login name if that's what you'd prefer.
There is a bug where the first time you log in the system shows your login name instead of the friendly name, and that may account for the impression that your username changed without your intervention.
Let me know if you have any trouble getting your name set up.