I wonder if you have IST - Inappropriate Sinus Tachycardia, apparently it is quite a rare arrhythmia but harmless (although can be disabling in relation to frequent symptoms). From research I have done myself I have learnt that it is not completely understood yet but may means that basically you are very sensitive to the production of adrenaline and therefore your heart level raises inappropriately (you heart rate increases by too much) in relation to the action you are carrying out or on minimal exertion like getting up from sitting and going up stairs etc. There are other symptoms like palpitations, tachycardia, fluttering sensation in chest, skipped beats dizziness/giddiness, shortness of breath on exertion and fatigue. It is also my understanding that it doesn't necesaary show up on an ecg as you are in sinus rhythm. My own cardiologist thinks I may have this because of my own history but wonders if there might be something else going on as I have other abnormality which do show up on ecg (Incomplete right bundle branch block, inverted T waves and ST segnment shift (flattened and depressed). Are you on an betablockers? or has anyone suggestion having cardiac ablation for your symptoms?.
Hope this helps a little, sorry to heear you are struggling with this. Shel