I can't help but be irritated

The day hasn't been perfect, but it hasn't been bad. There was nothing blissfully happy about it and yet it was just another day. But this evening I've had an underlying current of anger and irritation and I can't figure out why. I know what your thinking, its not pms. I genuinely want to be angry and I don't understand why. I was reading with my little sister, like I do every night, and I had to generate all the patience in the world not to blow up at her. She didn't do anything at all. We had pleasant conversation and I don't think she really noticed, but even now I find myself irritated and frankly furious for no reason. I'm not mad at anyone, so the anger isn't directed at anyone but yet here I am beside myself with absolute rage. I've never experienced such irritation and genuine anger with no apparent cause. I find it even difficult to write because I clench my fists and lock my jaw so frequently... I'm not generally an angry person, in fact you could say I'm quite the opposite, I don't lose my temper very often, because usually no one can set me off to the point where I'm genuinely angry. I don't understand it.