Being confused about simple addition problems

This morning I was very tired at the school that I work at. I was copying some notes from the overhead. This is what was on the overhead: 5+2+6=13. As I copied this on the paper, I thought that the signs must have been wrong because in my confused brain 5+2+6=16. It took me a few seconds to get it straight in my head. I figured out how I got sixteen instead of thirteen. For some reason, I was multiplying five and two instead of adding them. I'm not sure why I was doing this, but a brain is a really complicated thing to understand.