Most do not realize this but both //www.site.com and //sub-domain.site.com require specific DNS records. They need to resolve in the DNS system to a specific ip address and a host. Both records (www and sub-domain) require specific settings. Start by contacting your web host, gather info., then if not resolved, contact your DNS provider (they may or may not be the same organization). While you may believe that you do not have a sub running, often one is and a host or DNS provider is providing a redirect. So, we here at the forums are quite unlikely to affect a change, you need to please follow above and contact host/DNS provider.
You can use these tools to discover the host and DNS provider.