Telling my principal

I'm looking for some advice. I teach HS students, and spring break starts next week. I was planning on telling my principal that I'm pregnant when we return from break (I'll be 13 w 1d at that point). 
Today, I had to break up a fight between two male students in my classroom. Thankfully, I work in a school where this does not happen often, but today it did. I should have moved towards them to break them up and separate them from other students, but thinking about being pregnant made me step back from the situation. I was so relieved when the students stopped when I yelled for them to stop! 
Do I owe my principal a reason for my reaction? Should I mention something tomorrow, or not bother unless he asks?