I experience this, its because when you are sitting down doing nothing your mind is constantly thinking about your symptoms, and stange as it may sound, you might not even think that your doing it. The key to this (or so I find) is get up and do something, even if you dont feel like it, you need to do something that will take your mind off it. Remember this can be hard, just try to forget about it as much as you can. Hope this helps,
james