My business email address is associated with another Apple ID and it isn't my AID.

I own two domain names and have setup 1 email address for each domain. Today I attempted to setup a custom email address in iCloud.com using my work email address (self employed) thinking that I could stop paying for an email service and use iCloud instead.

I've added the domain to iCloud.com but when entering an email address for that domain, I get an error message that the email address is already associated with another Apple ID. Since I only have one Apple ID, I believe I may be a victim of hacking.

How can I dissociate my email address from the other AID?