If you have been charged for a membership, you must have an active Premium membership, you have yet to unsubscribe.
When you sign up for Premium, you accept the terms which include automatical renewal of the membership after the mentioned period - unless you unsubscribe it before.

Read the terms of your membership in "My Premium", which you find in your settings. Here you can also see the status of your membership, if and when it is set for renewal.
It is also here you unsubscribe your Premium membership.