Why do I get this Burst of Anger?

Sometimes when Im just sitting around and suddenly I get really angry and even the smallest things I don't like make me furious. For example I was chating with a friend and he was sending me some music and it got anoying because he would not stop and normaly I'd jusr delete it afterwards but this time I go so angry and I still didn't say anything but I threw my mouse on the ground (it's not broken thank god) and I really got pissed. Or when I paly quake 3 and I loose and normaly I'd just play again and NOT get angry but again... I really got angey and...again threw the mouse :D:D. Or if someone is talking to me and I just start yelling to him/her. Why is this sudden anger?? I'm only 15..I don't drink or somoke and I'm perefectly helthy with no "conditions".