That's a very common problem... I tend to have a cryspy face when I play, but then I relax as I get into the song... THat is especially true when I play a new song, or one that is more technically challenging. Something very funny: when I started playing, my wife told me she couldn't watch me playing as SHE would get out of breath!!
That improves too with experience as we better control the bellow action. One of friends recently told me that my music was great, but now I needed to improve my visual by better looking at the people I'm with when I play and SMILE. I am working on that right now.