If you canceled your pledge and were still charged, chances are you had a pending pledge. The creator you were supporting is most likely receiving payments "per post" and posted a paid post before you canceled your pledge.
Even if you canceled your pledge before the start of the pay cycle (the 1st of the month), pending pledges will still process because it is a charge for content you already received.
If you're uncertain whether or not you have any pending pledges due, check out your pledge history page. If any paid posts were created before you cancelled your pledge, you'll see them listed here!