Constant bad mood

Unable to get out of this bad mood, I do not know what is keeping me in it.
After school last night I went out with some of my class mates to have a drink, it was great, relaxed, laughed, complained, it was a good night out, never felt so comfortable and relaxed like I did in so long!!! Even a smile on my face.
I got home, checked my email and then I exploded again, I was so mad, upset, pissed off I just cant explain it.
Anyways I am not going over it all again, I am just trying to get out of this mood yet I don't know why I keep getting into it, there has to be something deep rooted here and I need to dig.
I have been looking over the issues that have been putting me in this mood, I have a little thought in my head that it has to do with my comfort factor, that I am trying to fit into something (a crowd) that is not comfortable with me, or me with it, yet I still try. I know I am a nice person, I can put up a front that anyone could believe, I know this because I am always holding a mirror up to myself to reflect and to see what other people see, scary most times but I don't want to offend anyone, even when I am cussing and complaining in my head I have a smile on my face but wishing a bullet would hit my in the arse just for an excuse to get out of there!