You can use Flow Logs with Athena to dig into your network traffic. Maybe you're polling the RSS feed too often.
VPC Flow Logs will capture the traffic going in and out of your instance and amount of bytes. Can also use Cloudwatch to monitor the traffic and set alarms for when the traffic goes above a certain threshold. "Data in" from the internet to AWS is not charged https://aws.amazon.com/ec2/pricing/on-demand/#Data_Transfer
In addition to examining VPC flow logs to determine network outbound traffic (DTOs, data transfer out), you may also want to examine network inbound traffic in your MongoDB hosting environment. See if any packets are lost or blocked in your network. This may give you an answer why Mongo Database is still 4.44mb in total. If you would like to find out more details with regard to the data transfer costs in AWS, please refer to: https://aws.amazon.com/blogs/architecture/overview-of-data-transfer-costs-for-common-architectures/
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Thanks a lot - I am polling it a ton, but because it's fairly high frequency. Wouldn't that be 'data in' to AWS though? and not count towards this DataTransfer? (the rep I talked to told me it was only data out of AWS that counted)
You have to send the request out in order to get a response. Even small requests can add up if it's constantly polling.