I know what you mean, I do that too. I don't know why? I take things really lightly most of the time, people are mad at me a lot for not taking things seriously enough. Then out of nowhere the tiniest thing happens and i'm so irritated... I don't know if subconsciously i'm actually mad from something that happened earlier and it festers although i'm not realizing it? Then when the small thing happens it just makes me crazy. Weird.
I say just ride the feeling out until it's done. It probably won't last long. I know its hard to get out of a funk, even if you watch something funny or hear something funny you're still kinda angry in the back of your mind. So yeah, just let the emotion play itself out until its over. Or do something that makes you happy and wait for it to take over the feeling of anger.