Roy It could be that you started in another plan, and then you tried to access a feature that was only available in a higher plan. The app would in this case ask you if you want to change your plan. When you then say yes to that, you would‘ve switched plans and your billing would‘ve started.
However, no way for us here to really know what you clicked on in the app. In any case plans are not switched or changed by accident, there‘s always a full explanation screen. As long as you read before clicking on buttons, nothing can happen really.
I would just ask bunq then, they can better see in their systems what actions you have taken. And they‘re also the right place to ask for a potential refund of any fees.