If you were charged after you canceled, here are some possible reasons.
You haven’t completed the cancelation process
Go to account settings (
https://account.nicovideo.jp/my/account) to check your current membership status.
You might have another Premium Member account
We charge membership/subscription fees per account. We have seen a lot of people using diferent emails, phone numbers and social media accounts such as Twitter or Facebook to sign up for Premium membership.
Try logging in with the information that you can recall to check your current membership status.