Facebook - question about usernames and privacy.?

[JaneEyre]

Help! My Facebook profile has always been set to private and the only information displayed was my name, hometown, school and year of leaving school. But there's been a change since yesterday - for some reason Facebook has changed my profile and is now showing my username. There is an option on my page which I didn't put there, it says "username:------------- public profile" . If you click on this my page comes up as it would for one of my friends. I never ever wanted my profile to be public and thought usernames were only for friends of mine to know if I tell it to them. It's so annoying as I didn't do this and there doesn't seem to be anyway to change it back. It says in the privacy options which I've gone through again and again to try to solve this, that I can only change my username once. The one I had is useless to me now but if I change it the same thing will probably happen again and it will be visible to all. Does anyone know why this happened and what can I do to make my profile private again?
Thanks. That is pretty worrying as all my details are now public and I can't do a thing about it. Does anyone know if there is a way to delete the username.