i had a problem similar to this when i was younger, i still have it now but not as bad, turns out i have an irregular heart beat, low blood pressure and a heart murmour, so one minute my heart rate will be normal, and the next it will be through the roof for no apparent reason and like you said, it is noticable, because it makes me feel drained and somewhat exhausted, and i do randomly have a tight chest at times, sometimes it feels like it's hard to breathe... i havn't been to the doc in years about it as it's gradually getting less and less common, but i would suggest you seek medical advice jsut to be on the safe side.