Let me preface this by saying that I'm an absolutely untalented musician, as well as bass player.
I often notice that when I play and concentrate on the rhythm, especially if there are 1/16ths making my life hard, I have a bassface. I notice it because I feel the tension of my facial muscles, and imagine that I must look pretty damn ugly. But I can't drop it, I just keep that bassface until I finish rehearsing/playing.
I also noticed that I have various "versions" of bassface, some more relaxed/enlightened, others more demonic and grotesque. And the latter seems to be the case more often as the years and months go by.
Should I be worried?