AWS Data Transfer

Hello.

I'm reading that AWS Data Transfer (1.0 GB are always free per month as part of AWS Free Usage Tier (Global-DataTransfer-Regional-Bytes)) is the data that is moved between AZ's from your EC2 instances. However, all my instances are in same AZ. Why did I get a notification that I have used 89% of that data within one month? I tried the cost explorer, but my cost explorer dashboard is empty. Then I followed these instructions https://aws.amazon.com/blogs/mt/using-aws-cost-explorer-to-analyze-data-transfer-costs/ and still nothing shows up on that cost explorer, and I dont know how this 89% of usage has been calculcated?

Do you know which AWS resources EC2 is communicating with?
For example, are you communicating with RDS with Multi-AZ enabled?
In that case, it is possible that the primary instance of RDS is running in a different AZ than EC2.
You may also be able to see more detailed information by looking at the "Data Transfer" column on your invoice.
https://docs.aws.amazon.com/cur/latest/userguide/cur-data-transfers-charges.html

Second question: Even though this is super weird situation, what is the price if I continue using my instances and it exceeds this tiny 1.0 GB data transfer, I tried AWS calculator and cant find it.

There should be a fee for this as detailed in the document below.
1GB costs $0.01.
https://aws.amazon.com/ec2/pricing/on-demand/?nc1=h_ls

Data transferred "in" to and "out" from Amazon EC2, Amazon RDS, Amazon Redshift, Amazon DynamoDB Accelerator (DAX), and Amazon ElastiCache instances, Elastic Network Interfaces or VPC Peering connections across Availability Zones in the same AWS Region is charged at $0.01/GB in each direction.
IPv4: Data transferred "in" to and "out" from public or Elastic IPv4 address is charged at $0.01/GB in each direction.
IPv6: Data transferred "in" to and "out" from an IPv6 address in a different VPC is charged at $0.01/GB in each direction.
For data transferred between a Local Zone and an Availability Zone within the same AWS Region, "in" to and "out" from Amazon EC2 in the Local Zone is charged at the following rate:

If you are transferring data between 2 VPC in same AZ, there is no additional charge is applied, but cross-AZ and between 2 VPC it will cause you charge to transfer data.

You can check if it relates with your situation: https://aws.amazon.com/blogs/architecture/overview-of-data-transfer-costs-for-common-architectures/

Hi guys, thanks for all your answers. So I have created my 5 instances on same AZ eu-central-1a as seen below

And I have created 3 EBS volumes as below:

All these instances are in the same VPC. I just used them for about 5 hours (at the same time, deploying a solution) , and then I got this notification after couple hours. And no, I'm not using RDS Service.

Still not sure if this is a miscalculation from AWS , should I open a support ticket?

All I can see at Billing and Cost Management is this under Bandwidth section:

$0.000 per GB - regional data transfer under the monthly global free tier 0.879 GB