It's official, I have anger problems!

Okay, something happened this morning and it has left me boiling with fury, like a frothing kettle. There is this woman whose head I would LOVE to cave in! Seriously!
Let me start at the beginning...........
I went into my local shop with my three year old son after collecting him from preschool. I was getting stamps and something tasty for him. I went to the counter to ask for some stamps and told him to go and pick something. He brought a little chocolate bar up and placed it on the counter. The cashier told him to put it back, thinking he wasn't allowed anything and he did. I didn't say anything for a moment as I was stunned by this. When he was over by the sweeties after returning the bar I told him it was okay and to get something. So he brought a different thing up to the till and began to cry. I hugged him and began to feel angry at the woman. I went back to the sweetie display to get the thing he had chosen first and he followed me to say that he wanted his second choice, still crying. So I bought the box of candy stick he picked second and my stamps and left. All the way home I was boiling with fury. I wanted to go back in that shop and shout at her, call her insulting names and kick up a fuss. Maybe even punch her. I had the urge to vandalise parked cars that my sons and I walked past on the way home, just to vent my fury. But I held it in. I definately have anger issues! Some of you might say I don't cause I have the strength to hold it in and not act on my anger. But what happens when the day comes and something happens where I can no longer contain it and lash out? Where will I be then?
Don't answer that. I can imagine where I'd be. It concerns me that something so trivial can get me so worked up. I guess everyone has theri triggers. In fact, I am still a little bit pissed off right now. But I will cool down soon enought I guess. Just so long as I don't go back in that shop later, when I collect my older son from school.......