I m having fluttering in my chest cavity, no shortness of breath, no pain....but feels like anxiety or like the feeling one gets before speaking before a crowd. It started about 2 weeks ago, no life style changes, no real anxiety, but I am feeling it several hours a day. Not taking any meds, 50 yr old female, relatively good health, non-smoker. What could cause this? Any thing I should be concerned about?