Heart racing and beating too hard

I'm a 14 year old girl and I have recently been suffering from problems with my heart which have been getting me worried. Every time I stand up or do any kind of exercise(walking down the stairs) my heart begins to race and I can feel my heart in my head and through my chest. While I was walking down the road it was so bad I could feel my heart through my chest. I am quite sporty and a healthy weight and have no other medical conditions. I have, however, suffered from a similar situation when 9 but resulted in no conclusion as to what had caused my heart to race. I have had an ECG and blood tests but so far have resulted in no cause. Seeing as the ECG was me lying down on a bed relaxed I doubted that there would be any change in my hearts activity. As I am a keen sports player I am desperate to know why this continues to happen and becoming increasingly nervous and worried about the cause of my heart pounding. If anyone has experienced this and knows why this is happening please help!! Hoping for an answer, Emily

I have a type of dysautonomia called POTS (Postural Orthostatic Tachycardia Syndrome) its most common in teenage girls. It causes an increase in heart rate when standing. For me, my heart rate is about 80-90 while laying down, but 120-150 while standing. Doctors don't know a lot about it, and I had to ask them to check me for this, and after being told that I had an "anxiety disorder" for months, I finally proved that I had a legitimate health issue. This disorder is one you will probably have to learn about yourself, and the doctors probably won't think to test you for. Just try to take your pulse after lying down for a few minutes, then stand for one minute and take your pulse again. if the difference is >40 bpm then it looks like this may be your issue. check your blood pressure changes too...

good luck (:

ps-if you do have a significant change upon standing, or have any other questions, feel free to message my account (: