Hey there @AlexOprea,
As Spotify looks to improve the overall Spotify experience, they’re now automatically generating usernames, hence the random letters and numbers.
I'm adding this help guide on how to change usernames in case you need further help.
On top of that, you can "Sign out everywhere" on your account page online, Re-check FB permissions on your FB page and log back in using your FB details.
Note: It might take up to 24hrs so give it a bit of time.
Hope this helps 😃