@Brenton0 I checked your listing and you have Instant Book turned on. With Instant Book anyone can make a reservation without your review or approval ahead of time. Seems like this guest just went ahead and booked.
You can try to turn off Instant Book (some new listings have it set as a default and host are unable to change it) by going to Manage Listing > Calendar and More then on the left hand menu click on Booking. Under who can book instantly, click on "no one..."
Also, you will see other IB choices that you may employ if you are not able to turn IB off. Be sure to check your Availability settings as well, specifically how far in advance in days a guest can book so you are not caught off guard.
Hope this helps!