After I changed my subdomain (e.g. from a.zendesk.com to b.zendesk.com) and I mapped it to a new external subdomain (e.g from a.mycompany.com to b.mycompany.com) I now get 403 Forbidden from cloudflare whenever I try to visit b.zendesk.com (or b.mycompany.com)
I cannot log into my account to check my settings or change anything as I cannot even login to access the admin settings.
I found one URL that does not redirect me right away (b.zendesk.com/access/normal) but once I log in it redirects me to b.mycompany.com and the same 403 page.
Has anyone encountered this before? Or what remedies I might have? I cannot even log in to contact support from my account.
403 page in question:

Hey David,
Sorry to hear you're experiencing issues accessing your account! I'm going to create a ticket on your behalf so our Customer Care team can look into this further with you.
You'll receive an email shortly stating your ticket has been created.
Cheers!