What does it mean when guys clench their jaw?

Basically, there is this guy called Chris who I am stupidly into and for a while he was into me. Since then, we have been thrown into an unexpected situation where nothing can happen between us and nobody can know that we are or ever were into each other; I choose not to disclose too much but, put simply, he would probably lose his job. I don't know if he still likes me or anything, we haven't had the opportunity to speak about it, but we still get along very well. Anyways, today I was in a room with Chris and a load of my friends (a professional environment) and one of my good male friends who I have no romantic interest in came up behind me and started hugging me. He was just messing around but I looked over at Chris and he was already looking at us, then he just looked away and I noticed his jaw was clenching. I've seen this happen in other people and I know it can be a habit but I've never seen Chris do this, and I just wondered if it could be anything to do with me. Was he mad or something or was it to do with the situation I was in? What did it mean xD
What does it mean when guys clench their jaw?
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