Add on domain url looks like a subdomain

I created an add on domain and installed wordpress. It is in the public_html/mydomainname folder. Yet, when I log in to the wordpress admin from my browser and the actual website the URL is mydomainname.myprimarydomain.com. None of the other websites at the same level in my folder have done this. If I try to log in from mydomainname.com/wp-admin it redirects to the version above that looks like it is a subdomain. I tried a redirect but that doesn't solve it. How can I correct this before getting too far into developing this site? Thanks so much!!