Thanks. That's good to know. I might have hit my arm when I was sleeping. Or maybe from me being on the computer. When I use the mouse, my arm presses on the desk.
I'd say give it a couple of weeks. If it was due to an injury it will have gone away or nearly have gone away. It doesn't sound like skin cancer at all.
Often we injure ourselves without remembering. Sometimes in our sleep, sometimes from lifting something heavy, pressing against something for too long.
If you get other unexplained bruises you might want to head to a doctor, but for just one I wouldn't worry too much.
Take care.