I've been cutting myself lately and I don't know why [Long plz Help]?

My mum took my sister to piano lessons. So I was alone since Mum and Dad got a divorce. I'm not at all sad about it so that can't be the reason why. Anyway I was pretty upset with my mum because I has waited 3 Weeks to get the internet back and she told me I was not allowed to use the computer.As soon as she left I ran into m bedroom and took my in-haler as I have a very bad cough but I had too much and remembered that you could p out from overdosing on Ventolin. I did however lay down and couldn't move it was too hard for me to breath and then I tried to get up. Once I did and took a few deep breaths I over dosed again. I don't know why. I just felt at peace. Then I decided that it wasn't a good idea. I ran into the kitchen and got out a knife and went into the bathroom. I cut across my stomach, my lower back and my waist, It wasn't deep but it drew blood. It also wasn't the first time I've done this. I' haven't been acting myself lately either, I'm not sure if it's just because I'm a 14. My mother says it's just normal teenage behavior. I haven't told her I cut myself though. I aslo have thoughts of killing people and have actually planned a murder and ways of transport, cleaning up after it, but not where to go from there. I think i'm just over reacting but I want to be sure. I am sure as not telling my mother though, I told her I thought there might be something wrong with me but she just puts it off with a simple. "Oh no darling, if you where I would know don't be so silly, stop acting so silly your fine there's nothing wrong with you". That just makes me sick,. Thats what makes me angry. To simply treat me like an infant when I am obviously concerned for my health.