My boss complained that I was sitting down

As a teacher, I am constantly on my feet, even in class. I never sit down, constantly walking around, monitoring classwork, helping students. Even when I was 9 months pregnant I was walking around the class.

Today, I was sitting down for 5 mins as I was writing down questions on the laptop to display to the class. Apparently our principle was standing outside observing me. My head of department came to me later and said he made a comment about me sitting, which I explained the situation.

I’m just very annoyed because I NEVER sit in the classroom. He caught me at the wrong time. Now surely he’s going to doubt me as a teacher. I feel so upset now.