Your premium membership may have been canceled due to one of the following reasons:
You might have an additional premium account
If you have multiple accounts, go to your account settings(https://account.nicovideo.jp/my/account)
Then, check the membership status of the account you're currently logged into.
Your PayPal payment didn't go through or was declined
When you don’t have enough deposit or your credit card is getting declined on PayPal, your membership will be canceled automatically.
If PayPal fails to make a payment at the beginning of the month, it will make another attempt five days later and repeat the attempt two more times. However, the account will be deemed unavailable after a failed third attempt. Then all the subscriptions will be canceled automatically at around the 11th of each month.
For example,
1st attempt: At the beginning of October
2nd attempt: October 5th, payment failed
3rd attempt: October 10th, payment failed
Subscription canceled: October 11th
In this case, your premium membership and channel subscription will end on October 11th. You are being billed until September and that will be your last bill.
Please contact PayPal support for any further questions.