I've got the same problem sometimes when I'm doing something difficult which needs alot of concentration, or if I'm just living myself totally into the music.

I think the first thing is because you - like when you are picking fast you sometimes tense up in the arm or something - just doing it in your mourth ^^.

And i think the other thing is that, you and the music becomes one, and your face is then showing some of those feelings found in the music.

but personally i don't think it's ugly or anything, i only think it's cool when people i so much inside the music that they do things like that.