It is your Browser cache issue. clear & close your browser or perform the following, If you are using IE or Chrome
IE -> Internet options -> security -> (Select internet/intranet/trusted sites, which ever is applicable for your site) -> custom level -> user authentication -> Logon: Prompt for user name and password.
If you are not able to find the site level, do it on all three.
This setting will prompt for the user name and password, every time, so that you can key in the required credentials.