Increased heart rate suddenly. Issues?

Last year i experienced heart paliptation suddenly for 4-5 in the span of 2 months. I taken ecg echo chest xray blood test and tmt multiple times.. all the time results was normal but BP only little high i.e 140/100 So I am taking BP tables for last 6 mnths.. as doctor advised.. now BP came under 130/90. after that last 6-8 months i feel completely normal. But today suddenly again i got heart palpitation for 5-10 mins..(120+ beats per minute). I went to hospital and took ecg which is normal. (See attachment) I am working in IT field, working 12 hrs daily, for long days without leave.. And also thinking many things alone in room.. So what is my problem doctor. Is I really have any heart problem or any other health issues? Please advise.. Note: As i am typing is, i am normal now.. last 1 hr i was extremely nervous and shivering..