I just got a $3,000 water bill based on "adjustments" for water usage in December

Just got my May water bill and it was $80. Except, well, no–it was $2,922 because of an "adjustment" to my December water bill.

They are saying they originally billed me based on "estimated readings" back in December but after taking a manual visual read we used over 200,000 gallons of water that month. My wife and I are the only ones that lives on this property, and we have had no evidence of a leak. Even with a leak, is 200,000 gallons even possible?? Furthermore, are they allowed to go back and change the billing from months ago?

I'm at a loss for what to do. Do I get a plumber out to check every pipe to verify there are no leaks? Are there any steps I can take in challenging their readings?