You might want to consider if anyone else could have had access to your account, or a live NowTV device or app of yours, and bought something. Which, without a Payment PIN in place, is something even a small child could do.
So, go to My Account/Bills & payments. This should show you what the payments are for.
If you then still disagree with them, or if there is a discrepancy between what you have been charged and what you see there, proceed as @gavs82008 suggests.
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.