I get random episodes where my heart begins to race, I tense up, and it's a bit difficult to breathe. Usually when this happens, I'm not doing anything that would normally cause someone's heart to race, I could just be lying in my bed. It happens fairly often, at least once a week. It doesn't stop me from being able to do what I need to do, but it is very uncomfortable and I was wondering what it might be that is causing this?