It does happen to me at the therapy appointments. Sometimes it is so bad,not only am I sitting there just shaking, but my teeth are chattering too. The therapist usually tells me to breathe - which sometimes helps a little and sometimes not at all. I do not know why I do it. Rationally I recognize there is no danger and I can leave and so forth. But there is some part which is somehow terrified of something. It used to happen (for about the first year) every week. Now it is sometimes less as in I do not physically shake as bad but internally it feels awful every week.