Is there something wrong with me, or am I just exaggurating things?

I don't know whether it's just me being a little hypochondriac here, but here goes:

I am fifteen years old and a few weeks ago I started feeling quite 'down'. I keep asking myself stupid questions like, 'What's the point?' and 'Why should I bother?'. I have started losing sleep because I just can't get to sleep; sometimes I just sit and watch people walk up and down my street out of my window until I reach the point where my eyes are itching and I'm nearly crying for lack of sleep.

I have been overwhelmed by a feeling of emptiness. I used to be a happy person, I hardly ever cried (you could call me almost unemotional) but now a lot of the time when I have free time (which is quite a lot) I feel the need to cry. It's not like I'm upset, it's just I have these big rushes of almost anger-like feelings, that make me really sad. I think back on the past a lot, and this doesn't help. Recently, this became so bad I started cutting myself - not much, but enough to wake up in the morning and be ashamed with myself.

However, when I have something to do, like school work or when I am with my friends, I don't feel as bad. I act like a normal person; I laugh and I can concentrate and I generally have a good time.

I have a supportive family who have noticed I have been feeling 'down' but have dismissed it as 'my hormones', because I try to be happy when they are about so they don't get upset.

Is it just my hormones? Am I exaggurating the situation, or do you think there's something wrong with me? It has only been happening for a few weeks, so I might 'get over it' soon. But I somehow can't see that.

I'm sorry about the length of my question; it needed explaining. I needed to get this off my chest.

Thanks a lot.