I think there are really two seperate questions here.
There is a huge difference between chronic anxiety and a panic attack.
Chronic anxiety as the name suggests is an almost constant condition, probaby lasting for days at least, whereas a panic attack is an acute fear and shorter lived...it just felt like a lifetime.
My panic attacks were usually caused either by being in specific places or situations, or by an anxiety build up from knowing that I was going to be in those places. The latter usually resulting in not going to that place. Like JKRsGirl being in a queue to pay for something was a guaranteed panic until I found my solution.
Other times, and probably the worst was the one that took me completely by surprise, anywhere, anytime and totally unexpected. Usually shorter(minutes) but much more intense.