Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. First of all no need to worry for heart diseases and Pulmonary embolism because of recent normal nuclear stress test. This kind of sudden onset breathing difficulty in sitting position is seen with undiagnosed stress and anxiety. So better to consult psychiatrist and get done counselling sessions. Try to identify stressor in your life and start working on its solution. You may need anxiolytic drugs too. Don t worry, you will be alright with all these. Avoid stress and tension, be relax and calm. Hope I have answered your query. Let me know if I can assist you further.