I am guessing you noticed the charge for next month by going to My Account > Bills & Payments ?
There maybe a glitch or a quirk on the NOW TV system where i notice that when i am on an offer, the day the offer renews for another months free offer there is a charge on that particular day, then when i check the Bills & Payments section the next following day the charge reverts back to zero has it should be.
Also, there was no money taken out of my bank account looking at my bank statements.
What i would be inclined to do, is cancel the month sports pass again following the step by step on screen instructions (approx four steps) until you get the green thumbs up cancelled message.
Then jump back to Passes & Vouchers section and see if there is a red cancelled message next to the month sports pass and maybe take a screen shot of it (hopefully you will also get an email cancellation notification from NowTV).
If the red cancelled message is already there and you have received the email cancelled pass notification then obviously don't bother trying to cancel the pass again.
After that pop over to live chat and explain to them what you are seeing on your account and if they do take any payment today or tomorrow for the extra month pass that you cancelled then request a full refund.
You can reach live chat by opening this link page below and select the most appropriate help topic or nearest topic to your issue, then once you get to the end of the process, scroll towards the bottom of your screen page and the click the drop down arrow below Get in Touch to find the live chat button.