Yeah, right through high school I had to hear, "what's wrong" and "are you okay?" when absolutely nothing was wrong! Well, nothing right there and then was wrong but I think all the underlying tension showed on my face.
Maybe has to do with ISTJ's not being very in touch with their own feelings, a lot might be going and everyone else can tell, except poor unaware ISTJ!
its in the shoulders and neck line, relax the posture a little.