I have a condition called "POTS", which stands for postural orthostatic tachycardia syndrome. It basically means when I stand or walk, I have an extremely elevated heart rate. My resting HR can be anywhere from normal up to 110, and much higher when nervous even if I'm lying down. When I stand, my HR increases immeadiately and drastically, going up to the 140s or 150s. See if you notice an increase of more than 15 or 20 beats between sitting and simply standing up. If so, you may have POTS, which can be confirmed by taking a tilt table test (ask your doctor about this.) Are you also dizzy along with the heart rate? Or having pre-fainting spells? There are many symptoms associated w/ POTS...it's fairly rare, and fairly benign, though just another idea to explore. Good luck!