Tachycardia

I am 21, 5’5″ 135 pounds and in most ways average.  However, when I do any form of exercise, my heart goes from a baseline of about 70 up to about 140 just walking at 3 miles an hour, if I run it goes up to between 200 and 220.  Recently I’ve had an episode where I had to go to the emergency room because I had a baseline of 170 laying down, it didn’t go down with four bags of fluid and laying down for eight hours straight while they tried to figure out what was wrong.  I was admitted for observation and eventually it went down to the 120s and I was then discharged.  Have you ever heard of anything like that before?  I’ve always been very healthy, minus the heart tachycardia and they said that there was no arryhthmia with the continuous telemetry.