adding subdomain broke non-www

I recently created a subdomain for a website on my linode, and it made the non-www of that website display a 403 Forbidden error. It used to work fine before I added the subdomain.

I'm running Ubuntu 10.04 with LAMP with virtual hosting set up. I added a new file to my apache sites-available directory called subdomain.mydomain.com with this info (along with error logs, server admin, etc):

 <virtualhost 173.x.x.x:80="">ServerName subdomain.mydomain.com
     ServerAlias www.subdomain.mydomain.com
     DocumentRoot /srv/www/path_to_files</virtualhost> 

The file for mydomain.com has virtually the same info:

 <virtualhost 173.x.x.x:80="">ServerName mydomain.com
     ServerAlias www.mydomain.com
     DocumentRoot /srv/www/path_to_other_files</virtualhost> 

I added an A Record for the subdomain in my Linode DNS Manager, and enabled it in apache. Now it works, and so does www.mydomain.com, but mydomain.com isn't.

I can't figure out how to fix it. Help!

Thanks in advance.