I think it is not possible to change username, change your password is possible but change username is not, so you should create new account now if you are looking for some other username.
It may happened due to wrong password, now you may try to change your password by using you security answer. the other possibility is that may some one hack your account so this problem can come.
Some times its happened, I think once, you have to delete all your messages from inbox, the inbox will be refreshed and will no notification show you again.