Why do I feel so empty, like I'm depressed?

About a month ago, I changed and didn't really notice it until now. I used to make jokes and play with my cat and work on my writing with full on enthusiasm, but now I feel so empty. I go to school in a daze, and I feel as if I'm looking through a lens. I can't concentrate on anything and my grades are starting to slip. My parents think I'm just going through some sort of "stage" and try to talk me out of it. Usually I feel annoyed when they talk to me like that, but now I just don't feel anything, I just sit there, mute. I can't sleep at night but that's normal for me, because I have insomnia, but I feel even more tired than usual. When I first noticed this, I thought that I was just sad because my moms never worries about me, just me brother, nor does she encourage my passion for writing, she just tells me that there's no way that my book will be published. I don't think it's that now, because she's been encouraging me more but I just feel worse. I've started getting bad headaches too, behind my eyes. I considered that I was depressed, but I'm only in grade seven, why would I feel depressed? I did really, really like this guy at my karate class, but even he doesn't get through the void in my life now. Why do I feel like this? How can I get better?