PLEASE!!!! boss keeps telling me to work on my composure but I don't understand why?

One day my boss pulled me aside and told me that apparently I had a conversation with his boss saying that "I look mad all the time." What bothers me about this was that I actually had this conversation with a coworker, not with my boss. I guess she heard it. That day my wisdom tooth was actually coming in and she asked me why I look sad and I told her I look sad recently because of it. I was completely baffled, because I go out of my way to not talk to her. The reason behind that is because she's actually best friends with my old boss that fired me..so I just really try to stay low key.  

I told my boss that it must of been a misunderstanding. That I would never describe myself as a mad person. Nonetheless I would never have a conversation like that with my boss. 

He told me that what I need to work on is my composure at work. That I look angry sometimes. But it's funny it was never brought up before. And sometimes when I'm focused I have a serious face. I spoke to a couple of coworkers and they're actually surprised. I never let me emotions show. If I do make a face it's when I'm empathize with a customer. So basically my question is how do I got about this? Are my facial expression a sign of not keeping my composure? I'm not throwing a Tantrum. Just need some clarity