Why does my heart start racing suddenly?

i am 21 years old and i was diagnosed with sinus tachycardia, i am taking beta-blockers to treat this. my question is why does my heart start racing all of a sudden. it happens when i don't make any effort at all, all i do is sit and watch a movie for example and my heart starts racing and starts beating really hard (like it's jumping out of my chest). i had a ekg holter put on and i caught one of those moments, i practically had about 70-80 beats per minute and then all of a sudden my heart started racing and it went up to 98. this happens to me at least once a day without making any effort. i haven't had any alchool or caffeine in the last 3 months so that can be ruled out.
i had a thyroid test done and it's not that. i also had numerous ekgs done and they found nothing. i've seen a few doctors and they told me its nothing dangerous and they sent me home

[ljd]

Hi,

I was diagnosed with sinus tach 2 years ago.  Like you it would come on at no specific time.  On my holter I reached 140 bpm.  I take Toprol daily.  All I can say is that it does take a while to get used to.  It took me a good 6 months to actually accept it and deal with it.  Hopefully the beta blockers will work and you can get back to some kind of normalcy.

[RNRita]

Hi there.  In the cardiac world, we wouldn't even say 98 beats per minute is tachy.  My resting heart rate when I was in my twenties was 96.  I was put on a beta blocker and it took it to 72.  That was 30 years ago.  If I go off the beta blocker, it goes right back up.  Amazing, huh?  The only problem is that I have no stamina and have always gotten tired easier than others.  Oh well.  72BPM is much more comfortable.

[DJBecks]

my normal pulse is 70-80 beats per minute. when my heart suddenly started racing my pulse went up to 98 beats per minute while on beta-blockers, i dont even want to know how high it would have gone if i wasn't on beta blockers.