You can login to the receipts page on your account and click "view" next to the receipt to see why you were charged: https://www.spotify.com/account/subscription/receipt/
If you wish to dispute the charge,
I suggest you contact support so they can help you https://support.spotify.com/contact-spotify-support/
If you get an automated reply email telling you to check the help section or the community, you need to reply back to it, even if it's from a no-reply address.
Support usually replies within 24-48 hours.