I REALLY don't want to get into this but the word Agnostic has been bastardized from it's original meaning quite a lot.
The term "agnostic" is not a position, it's a world view. The term states "I think knowing if the supernatural exists is unknown".
The problem is that it's not specific to a God. You can be a Christian and be agnostic toward Big Foot. Or be an Athiest and be agnostic toward dieties. It's why there's an expression called an
"Agnostic Atheist" who does not believe in God but thinks knowing if he exists is unknown. By this line of logic almost every Atheist is an Agnostic. There are also
agnostic theists who believe in a God but also think knowing if he actually exists is not knowable.
The point I'm trying to make is you either believe there is one or you do not. Saying "I don't know" isn't called being agnostic it's called being on the fence. "going through my athiest phase" is not a valid statement because the only way to not be an atheist is to become a theist. This is similar to saying "I went through my heterosexual phase. But now I'm more of a sexually curious person" when the latter statement doesn't reject the former.
tl;dr both terms are not mutually exclusive.