Please help, is this a normal part of being a teenager or depression?

I'm female and 16 years old. I've been feeling down for ages, I have odd days where I'm in a really good mood but by the end of the day I start to feel upset and I don't know why. I don't know why I feel down so it make me think its just part of being a teenager. In November, 2011 I was at my lowest. I stayed in my room the entire month. I started to cut my wrists but I didn't let myself get out of control and it wasn't that noticeable. I started to feel a bit better after a few weeks. My parents are worried about me and they think there's always something bothering me. They ask me whats wrong but I wouldn't know what is actually wrong and just say nothing. I loose my temper very easily lately and once the temper stops, I cry. I loose interest in things easily and I can't hold a conversation anymore. When I'm with my best friend or some friends, I feel better but it doesn't last, The past three nights I haven't slept right. I'm exhausted now and I can't go to sleep. I've been getting awful headaches the past two months. I cry easily. I cry more than once a week, when I'm alone. No one knows how I've been feeling. I don't want to tell my parents because my dad will just tell me I'm looking for attention. I don't know what to do, I don't know if this is a normal part of being a teenager?