I'm not really sure, it's just nailed me like a ton of bricks. I'm thirteen and feeling depressed/sad, and I can't pinpoint why. It's just odd.
I'm not wanting to dismiss your feelings as something meaningless, but there is a good chance its mostly just down to puberty.
A whole lot of growth and hormone change can leave you with some wild moodswings and feeling quite stuffty. It's normally nothing abnormal. I went through it. I'm normally quite an upbeat person, but even I was pretty moody and sad in my younger teenage years.
It all gets better though. There might be a few tears on the way, but you come out the other side feeling much more grownup.
Just keep positive wherever you can, but if you get down, don't worry about it. It's not worth beating yourself up for.