It appears that the domain name server has been switched. However, it only works with mydomain.com, (not www.mydomain.com) Why is this?
During propagation, some traffic will still go to your old web host, some will go to your new site. It depends what your local ISP has in their "cache".
Similar to your web browser's cache, your local dialup ISP has a cache which stores recently accessed domain names, images, and web pages.
If you accessed your web site www.yourdomain.com yesterday (before propagation) using your local ISP's dialup, then access www.yourdomain.com again today (after propagation) you will get the same information.
However if you DIDN'T access it yesterday using yourdomain.com, and you try today, your ISP will NOT have yourdomain.com cached, and you will see fresh information.
If you don't understand all this, you don't need to! Just know that everything is wierd during the propagation period, and it will all work itself out in a day or so.