If you noticed that your username changed unexpectedly to a string of numbers and letters make sure to take a look at this Spotify Answer. This will help you perform the necessary changes and privacy settings on your Spotify account.
Keep in mind there's a difference between the username and display name of your account. Here you can find more info on how to change your display name to a desired one.
Keep us posted! We'll be happy to lend a hand if you need more help.