How can I get my boyfriend to see my point of view when we argue?

I had to go to the emergency room today to get an X-ray of my spine. My boyfriend didn't want to sit there and wait with me. Instead, he wanted to go do things for his friends while I waited. When I was done, he was twenty minutes late in picking me up. He doesn't understand why I'm mad, and we keep bickering at each other over the smallest things. He thinks I don't have a reason to be mad, but I believe that I do.