To ensure we get to the bottom of this for you, can you share with us how you made the payment back in September? By credit card or online banking for example.
It's possible that the payment method wasn't instantaneous (as in the case of online banking) and that we received your payment after your billing cycle ended. Meaning it would explain why you still see your previous month as unpaid. Rest assured though that your upcoming invoice will show your payment and the date at which we received it.
To confirm that your account is up to date at the moment, you can verify your current balance on your online account. You should see a lower amount on your current balance than your monthly invoice. That would mean that this month's payment would be that current balance.
Hope this clarifies the situation and put your mind more at peace!