Jaw Cramp?
 

I'm gonna put this here, and I'm not sure if it does. Anyway, I sing in three choirs at my school. Yesterday in one of my classes, I got a cramp in my jaw, underneath my tongue. Last night at choir, I had to keep stopping because it hurt to sing. Today, I have practice for a more select choir, where we have a concert in 2 weeks. What can I do about my jaw?

You probably hold too much tension in your jaw when you sing. This is only marginally better than trying to hold tension with your throat (which can wreck your voice), and can end up giving you headaches and causing you problems with the joints holding your lower jaw to your skull. You should be holding tension in your diaphragm when you sing.

It's really worth getting proper singing instruction if you are doing that much singing - firstly, your singing will improve (control, breathing, probably range and tone as well), plus you won't hurt yourself and will enjoy your newfound ability more. It made a huge difference to me.