I agree with pat.
additionally--anxiety/stress sets off the fight/flight response. this will sett off physical issues too. what happens is anxious thinkers freak out with fight/flight which only perpetuates the problems. at some point fight/flight can develop such a hair trigger that it will go "off" for seemingly no reason at all.
what I said above is explained very simplistically. not only that the why's of anxiety is a complex process. there are no short answers.