getting booted back out to login screen

Hi All –

I have wordpress running on my sub-domain (ex: domain.xyz.com)

i cannot login to the wp-admin console by directly visiting domain.xyz.com – I can only login by using http://www.xyz.com/domain (which is the folder that the subdomain redirects to on my hosting server)

in addition, when I do finally get logged in, whenever I make changes to widgets or appearance, etc. – I get booted back out to domain.xyz.com and have to re-login.

any ideas on this? It appears to be not liking my subdomain instead of a normal www address.

thank you!!