Often when stressed we breath less deeply and subconsciously tighten our muscles..
That might be part of it.

Try breathing deeply and fully expelling the air - diaphragmatic breathing not upper chest breathing..
If really stressed the "corpse " yoga pose with deep breathing is very relaxing
http://www.yogabasics.com/asana/corpse/
video-
https://www.youtube.com/watch?v=YBe1nzunHJ0