I'm a teen, thought patterns have changed?

Hey I am 14, and I recently noticed that I become a little depressed at some point on most days, and I can't seem to shake it. Also, my mind feels kind of cluttered, and even though I can still think in a linear fashion, my brain, without me wanting it to, goes off on a bunch of tangents. I've also started thinking about why things are the way they are, how we know about certain things, and what humanity's place in the universe is. Also some things have started to seem like they don't matter, and all this is kind of scary because none of this has happened before. Is this all a natural part of growing up? Could it be because of changing brain chemistry that happens in adolescence? Should I see a psychiatrist? I just don't know.