A typo is just a spelling mistake. It's possible that when you changed your password, you entered in your username incorrectly, and so the username changed to. Generally to change your password you don't need to enter in your username.
You go to settings, Edit Profile, and if you can change your name you should see a box that says " Edit Username" If you see a text box, you can enter the username you want in and save it. If you don't it will say you can't change your username yet, and the date you last changed it. You can only change names every 90 days.