DataSource Controls :: Sql Server Update Password Not Working

The users of my webapplication are validated using database connection. This requires each user is a sql server database user. While logging their validity is checking by trying to create a connection using userid and password entered by the user. If connection opens successfully, it is a valid user. I also have a page in application to change the password, where sp_password is used to updated database password.

string connectionString = ConfigurationManager.AppSettings["DBServerName"];
//try to create a new connection dynamically using user id and password
connectionString = connectionString + User ID=" + userId + ";Password=" + password;
using (SqlConnection connection = new SqlConnection(connectionString))
{
connection.Open();
}

The problem is after i change password, and logout and try to login, i am able to login using old passwd also. The line of code which creates and open connection succeeds with both old and new password. If i stop and run the application again this works.