Sir I created a free tier account using credit card. I have only intention to use only always free products like e2 micro instance. I created one instance in the free zone us-central. They didn't charge anything for that. I deleted it the same day because I needed one fresh instance, but that other instance I created after deletion of previous one started using my credits?
So, why did this happen. Because logically I had only one active instance which is free as per their documentation