My subscription has changed from monthly to annual without my notice.

I've been billed for a full year without asking for it. 

Have been doing monthly payments for 1-2 years and suddenly my billing has been changed to annul without my consent. 

How do I get my money back and go back to monthly payments?

When I click "cancel plan" it says that my subscription will end in a year since I've paid for it, but that's not something I've agreed to since I never asked for an annual plan.