Charged for free tier account

You mention instances (plural), how many did you have? Free tier allows you 750 hours of small (t2.micro or t3.micro, depending on the region) EC2 instances, which is enough to run one continuously all month. If you run two then you have to only run them for 12 hours each day, and so on. Any instance type other than the basic t2 (or t3).micro will be chargeable from the off.

Also bear in mind you're allowed 30GB of EBS storage in total (not per instance) for the whole month.

Good advice on tracking free tier usage is here https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/tracking-free-tier-usage.html

More details of what you're being charged can be found in AWS Cost Explorer https://docs.aws.amazon.com/cost-management/latest/userguide/ce-what-is.html

If you feel you have been incorrectly charged the steps for opening a billing support case are here https://docs.aws.amazon.com/awssupport/latest/user/case-example.html

You can go to the Billing dashboard and see the detailed charges by Account and by AWS services as well. Please refer this link on how to view your detailed bill - https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/getting-viewing-bill.html

The bill of current month, gets generated at the end of the month.

Also, You can use the AWS Cost and Usage Report or the AWS Cost Explorer to check your usage of resources.