weak (or negative) atheism =
lack of belief in deities
strong (or positive) atheism =
belief that deities don't exist
agnosticism =
belief that whether gods exist is unknowable
An atheist is a person who doesn't believe in gods. It's that simple. I don't make any claims, so I'm a weak atheist. For the very same reason, I'm not an agnostic.
If you truly didn´t claim anything religious that would be called agnosticism.
No. Agnostics claim that the existence of God is unknowable. I don't even make that claim.
As an atheist, you claim that god doesn´t exist, and the inexistence of something is a knowledge. In other words you "know" God doesn´t exist.
You're talking as though my label dictates my beliefs rather than vice versa. You have no reason to believe I claim anything, so don't make stuff up.
Did I go wrong somewhere around there?
Yes. You failed to distinguish between weak and strong atheism. You failed to understand the meaning of agnosticism. You failed to understand that even strong atheists have varying levels of certainty.