Dennis,
There really is nothing to be concerned about here. Neither the POP3 or your account name has really been changed.
More than likely your ISP has made some software changes that affect both how you connect to the POP3 and how the POP3 server recognizes your account name.
Localhost is usually the default name for any Unix/Linux computer. More than likely the ISP has changed its DNS table to identify the computer named localhost as the mail server. Your account name is essentially the combination of your account name and the email domain name.
Hope this makes sense.