What is the full URL of the link in the email you received? You must save your login information in your browser, otherwise I assume you would have questioned how you logged into the site at all.

It could be a phishing scheme that attacked your saved login information then placed that on a dummy site in hopes that you may provide even more data.

EDIT: They could have sold / transferred user data... but I don't know how they would automatically authenticate you without using some previously stored data that you, maybe unknowingly, gave them access to.