Giving off vibes?

On Monday, we lined up to start hoi jeon moo sool, and I was surprised when the master came up and said, "I know you were frustrated on Wednesday," (that was the day I drove home seriously considering giving up the sport) and then he demonstrated the technique. I don't know that I gave off "frustration vibes" and I know positively that I didn't say a word, so then how did he know? Happily, at class I did improve on the first part of the technique, the grip. We will see how it goes. If I decide that it would be a waste of time, effort and money, I will tell my instructor that I will not be testing for a black belt.