Is atheism a belief system in someway?

[Ben Davis]

I thought Atheist meant does not accept the existence of a deity and agnostic meant a person was undecided about the existence of God(s).

No, that's what some theists want you to believe

Seriously though, this conflation of definitions is one of the mechanisms used to misrepresent atheists.

So to me the agnostic atheist is a contradiction.

...and it seems to have worked, in your case! In fact, neither being an agnostic atheist nor being an agnostic theist is a contradiction. In the case of the former, one is uncertain about the existence of god but doesn't believe; in that of the latter, one is uncertain about the existence of god but believes. No contradiction in either case.