I have just received a similar alert for $ 5b

AWS on their support data is reporting this:

Inaccurate Estimated Billing Data

Jul 17 3:03 AM PDT We continue to work to resolve the issue affecting estimated cost and usage data displayed in the Billing and Cost Management Console. We have identified the root cause as an issue with unit pricing within the estimated billing computation subsystem and we are working on a mitigation. The displayed billing estimates do not reflect actual usage and charges. There are no customer actions required at this time. Once the issue has been mitigated, we expect full resolution to take multiple hours as we work through recomputing the estimated billing data. We will provide another update by 4:00 AM PDT or sooner if more information becomes available.

Jul 17 2:07 AM PDT Beginning on July 16 7:38 PM PDT, we began displaying incorrect estimated billing data in the Billing and Cost Management Console. Our engineering teams are engaged and investigating root cause. We will provide another update by 3:00 AM PDT or sooner if more information becomes available.

Jul 17 1:33 AM PDT We are investigating issues with Cost Explorer reflecting inaccurate estimated billing data.