From my experience, this is usually related to a video, did you upload any videos around mid october?
I should warn: serving videos from the Media Library can very quickly eat up your bandwidth allowance. You might want to consider a dedicated video hosting service, like YouTube, Vimeo, or Wistia. If you do choose to host your videos on Prismic, keep a close eye on your bandwidth usage.
Thanks for the answer. We did start to use a video on our home page around that date. We'll switch to Wistia, and follow up here if bandwidth doesn't go down.