There are a couple of reasons you could see a charge:
If you're on a paid subscription, it will renew monthly or yearly depending on your billing cycle.
If you added a new team member to your plan, you'll see a new charge for every new member.
If you have more than one team, and you're the administrator, you may be charged for a new member that was added to any of the teams that belong to you.
To check, switch between workspaces and see if any new members were added.
Contact us at [email protected] to fix any other billing issues.