Ted here from Shopify, thanks for posting your query!
It sounds as though another merchant previously had that domain listed on their account. The domain expired and so you were able to purchase it but the domain was not removed from the previous owners Shopify account. Not to worry, I can help you with this but I'll need to email you privately to help authenticate your account and prove that you're the new owner.
Have you just opened the store? How are you finding Shopify so far?
Please keep an eye out for my email and if there's anything else I can help you with, I'm happy to help!