Part-time Zen Master

Tonight at work things went from flowing very smoothly where we were super slow - to absolute chaos.
Both of our registers stopped working at the same time (for various reasons) right during our peak hour - when the lobby was full and the line long. I was working with two people who are still training - both very capable, just not yet completely comfortable, of course.
This was a high stress situation. One of the most extreme I've faced at work so far.
Yet, I was absolutely calm. I didn't panic in the slightest. I wasn't overwhelmed at all.
When the craze was over I was so impressed and shocked with myself.
It was wonderful. I was a zen master for that entire half hour or so.

A customer and his four-ish year-old daughter were there during the chaos.
He'd been watching me for quite some time, but I honestly interpreted it as he could see how ridiculous the situation was and perhaps he was impressed by my zen, too.
Then - he said, "I know you're really busy, but would you mind taking a picture of us?"
I'm thinking, "sure, not a problem."
Then am swept away to another emergency before I can respond to his request.
When I finally get back to him, and before I know it - he hands his phone to his daughter, stands next to me and has her snapping a photo of... him and I together...? What the hell.
He then preceded to spend the next hour sitting at the counter, trying to catch my attention by loudly tickling his daughter and asking me questions.
I did my best to ignore him, but I still had to step into the back a few times to regain my composure.
I managed not to actually cry - which was a big deal in itself. And probably was only possible because I'm friends with a person I was working with & I felt supported by him.
I was still shaken up after the customer left.
Which left me incredibly frustrated.

How is it that I can be a zen master through uncommonly high-stress situations and yet want to curl up, hide in a corner and cry when a person treats me like, well... like they do all the time.

Someday I'll be able to be a zen master in both situations.