Hey SongsInMyHead

It could be TMJ, as Raynaadi said.

Link of info

I also grind my teeth when stressed, so I had a mouthguard made.

Have you had orthodontics before (braces or retainer)? Since they are sometimes used to reset your jaw (move it forward or backward to be in line) it can cause problems. At least I think that is what my dentist said...

Best to go to a dentist and find out, hope you're feeling better soon. I know the feeling, and it isn't pleasant!