When you make nameserver changes to your domain with the domain registrar, the changes will usually take 2-3 days to 'propagate' across the internet, depending on each ISP and how often they update the DNS records.
Some ISPs update their DNS records more often than others and receive changes faster. You may be able to access your site in a web browser but other people using different ISPs will not. Until the changes propagate to every ISP the results will be inconsistent.
You can start accessing your web site before propagation by using the shared IP address that is in your welcome email e.g.
http://123.123.123.123/~username
