regional data transfer - in/out/between EC2 AZs or using elastic

Take a look at this question and check my answer there - it's a very similar situation: https://repost.aws/questions/QU4KuYNjlWTqWrqMA1RyWLYQ

There are multiple reasons why you might see these charges, but the most likely scenario is that either you have few EC2 instances running in a region, but in different AZ, and they comunicate between each other (which would generate inter-AZ data transfer charges), or you have instances in the same AZ, but you use Elastic IP address (public IP) for them to communicate.