You can cancel at anytime, this is not a contract, it is just a normal monthly subscription which will keep rolling until canceled.
You can cancel your membership in your account area on the website if you purchased through the website. If you purchased through iTunes (Apple), you will need to cancel through iTunes.
This almost always happens if you do not cancel 24h before your next billing cycle starts. Another less likely reason is that you did not confirm the cancel. On your account page, when you press cancel, it will ask you to confirm you want to cancel, please click confirm if you mean to cancel.