Why is it always me?

I went to a sub job today that I had been prearranged for. Yesterday she trained me how to do some things. I show up and low and behold, someone else had taken the job. The employee that I was subbing for didn't request me, they just sent it out. So someone else picked up the job.
I got there and the secretary said, "I'm sorry, it's her call. She's the one on the list". The person calls while I am standing there and says that she is running late because of traffic. The secretary talks to her and explains the situation. She says "I'm dressed and on my way, I might as well come". WELL I'M DRESSED AND HERE! I wanted to say that but I didn't. I said "No problem, call me anytime", and gave them my card with numbers on it.
But here is the thing that REALLY chaps me: A while back the same thing happened to me, except I was the one whose name was on the list. Guess what! Yeah, I had to go home.
This whole thing really ticks me off.
The other thing is that yesterday I left some things there that I'm going to have to go back for sometime today (I didn't remember until after I left).
Hope things are going better for you.