While sitting I have had a very intense feeling that I was falling. The type of falling you would experience on a roller coaster. I know I am sitting in a stable position with no movement, but my stomach tightens and my head feels pressure. I do not ride roller coasters, because I do not like the feeling. It has happened four times, with two of them being in the last week. I had chest pain about three weeks ago, with pain to my jaw and throughout my back, but my heart stress test showed no blockages and the doctor gave me a clean bill of health. Should I be concerned?