Man, i think i going "nuts". Office is ending and the day is great, but i do not know why i felt all so anger inside of me. Like a time bomb is ready to expolsive.

The last time, i have this overwhelming anger, i hit my sandbag until i bleeded from my knuckles.( With external and internal skin peeled off, lot of blood then).
Can any one tell me why the sudden built up of such dangerous anger.I'm one of those "nice nice" guy that doesn't yell or lifted a finger at anyone before, but why this sudden built-up of uncontrollable anger.

Any one have the same experience of such emotions.