I am free tier eligible and still AWS charges me. The charge is for my t2.micro EC2 instance.

Free tier is only free for a single AWS instance up to about 700 or so hours of compute time per month. If you have 3 EC2-Instances then you have gained 2,100 hours of compute time.

@David is correct that free tier entitles you to 750 hours of t2.micro every month for 12 months. Any more than that you will be charged.

Free tier also entitles you to 30GB of EBS block devices per month. So if you've stood up four EC2s each with 30GB then you're going to be charged for the extra 90GB. Also be aware that I/O optimised volmes are out-of-scope of free tier.

The second part of your question is:

it is still increasing after I have terminated all my instances. Any idea on this?

• check that you have terminated all of the instances, and not just stopped them
• check that any volumes and snapshots have been deleted
• check that any Elastic IPs have been released

Scroll down to the foot of the right hand pane of https://console.aws.amazon.com/billing/home#/bills and it will show you everything that you're running in the current month that is billable, even though the total may be zero if you are still in free tier.

Expand each section and drill down to see which region(s) you are using resources in.

AWS Cost Management can also give you a view on this https://console.aws.amazon.com/cost-management/home#/dashboard

This document is useful https://repost.aws/knowledge-center/ec2-billing-terminated