Billing before the end of the free period.

There are a few potential reasons why you might be seeing charges on your AWS account before the end of your free tier period:

1. Exceeding free tier limits: The AWS Free Tier provides a certain amount of free usage for eligible services, but if you exceed these limits, you will be charged for the additional usage. For example, EC2 offers 750 hours per month of t2.micro or t3.micro instances (depending on the region). If you use more than this, you'll incur charges.

2. Using non-free tier services: Not all AWS services are included in the free tier. If you're using services that aren't covered by the free tier, you'll be charged for their usage regardless of your free tier status.

3. Misunderstanding free tier eligibility: Some services are only free for a limited time (e.g., 12 months), while others have different terms. It's important to understand the specific terms for each service you're using.

4. Charges for associated resources: Even if your EC2 instance is within free tier limits, you might be charged for associated resources like Elastic IP addresses, additional EBS volumes, or data transfer.

5. Viewing current month costs before free tier credits are applied: The AWS console may show running totals of your usage before free tier credits are applied. These credits are typically applied at the end of the billing cycle.

To understand your specific situation:

1. Check your free tier usage in the AWS Billing Console to see if you've exceeded any limits.
2. Review the AWS Free Tier page to confirm which services and usage amounts are included.
3. Examine your detailed billing statement to identify which specific services are generating charges.
4. Consider setting up AWS Budgets to alert you when your usage approaches free tier limits.

If you believe you're being incorrectly charged, you can contact AWS Support for further assistance and clarification.
Sources
Charged for AWS despite using Free tier | AWS re:Post
Understanding unexpected charges - AWS Billing
Avoiding unexpected charges after Free Tier - AWS Billing
I Have Use Free Tier Account But Charge Coming | AWS re:Post