Welcome to the community!
For any given billing cycle, you are charged the upcoming monthly fees as well as any overages you might have incurred during your previous month. So you are charged in advance for your monthly plan plus any usage above your allotments previous month. If your cycle ends December 8th, your bill would include charges for December 9 to January 8 (upcoming month) plus any overages during November 9 to December 8 (previous month).
Hope this helps 😀