For several months now, I've been having episodes of tachycardia. It doesn't seem to be preceded by anything; stress, physical activity, etc. Usually I'm just sitting on the couch. Suddenly, I can feel my heartbeat, and it's fast and kind of fluttery. I've taken my pulse during these episodes, and it's usually around 106. I don't know my blood pressure during them, but normally my blood pressure is low. Also, during the episodes, I feel kind of hot and sweaty or clammy I guess. I get dizzy and a little nauseous. They usually last about half an hour before returning to normal. What could cause these episodes?