I suddenly feel depressed?

Sometimes when I'm doing something fun on my own at the weekend - like art or something, but then I just feel miserable all of a sudden - or a lot of the time, I get sudden anger outbursts - like now, the spell check came up, and I was like ''GO F****** AWAY''. Just today I had to do a project and when it kept collapsing, I got furious and started crying! Or when my sister tells me ''wear some jeans, you look like a tramp'' I feel like smacking her face in. Why do I suddenly feel depressed? I'm not even a teenager yet. I tried posting this before and it said ''Oops, Yahoo has currently stopped working'' and I got really annoyed.