I get these kind of palpitations all the time. Last night I had some when I was in bed trying to sleep! Ive suffered with anxiety for about 5 years, had alot of heart checks and everythings fine. Anxiety/stress is a very powerful thing, it could easily just be that... and all the adrenalin your putting into everything you do might be tipping your heart rate up and up. By the sounds of it you might be making it even worse, now your worrying about it and your track running, it'll become a vicious cycle, everytime you train your heart rate goes crazy as your anxious about it the whole time?
Keep pestering your doctor. If you have had all the nessisary heart-checks and there all clear, theres no need to worry that there might be something wrong with your heart... practise breathing techniques and relaxation techniques. Try not to panic to much when its happening or it'll be even harder to make your heart settle down. When your young, there is nothing dangerous about your heart rate going up to 200.
Hope you work it all out soon. Keep us posted.