heart beat very fast

I noticed that my heart beat very fast at times. Sometimes I will be just driving or maybe just ate and lie down a bit. Sometimes it's like I am frightened for no reot feel ason and it scares me. I remember going to the doctor a couple of moths ago and he advised me that my pressure was a bit high and that it puts a bit of a pressure on the heart after he gave me a heart test. But I sometimes believe it's gonna stop beating. Bearing in mind I have no chest pains or it does not feel compressed. What can that be?