Palpitations of this type can vary greatly both in intensity and frequency. They can range from a mild buzzing sensation to a hard thump. This occurs because of the precise entry of the palpitation upon the normal cardiac cycle of the heart, either atrial or ventricular and the body's response to restore homeostasis as a consequence due to slight changes in blood pressure that may occur. Cardiac force is a mechanism used by the body to quickly restore pressure.  

When you say "physical discomfort" I'm assuming that you are specfically referring to pain. Typically but not universally, there have been reports of pain during benign palpitation events, characterized by sharp or jabbing pain that is very brief in nature lasting only the duration of the palpitation event itself and most often located mid-sternum or either left or right of midline just under the ribs. This is most commonly the result of pressure from gas trapped in the stomach or intestines pressing upward on the diaphragm that also produces a palpitation event.  

Since you've already undergone direct diagnostic evaluation by your doctor and no sign of cardiovascular disease was detected, it is not possible for such disease to suddenly manifest itself. It takes many years to develop.  

If the pain persists or the events concern you, however, then you should seek consult with your primary care physician. I simply remind you that it's absolutely impossible to gain some type of diagnosis or evaluation over the internet. Your experience with palpitations is much like any other condition wherein you become generally accustomed to the range of symptoms associated with it. I will share with you that pain typically associated with some type of cardiac event is more often dull in nature and tends to last for a while before receding, whereas the pain associated with GI difficulties or mere musculo-skeletal pain tends to be more sharp and acute, typically lasting only seconds with recurrence on an intermittent basis.

Best regards and Good Health
 

Ah, then you've nothing to be concerned about in this case as well. As I stated, cardiac force is a natural response by the body to compensate for brief changes in blood pressure. It can sometimes be quite dramatic, but nevertheless harmless in nature.

You'll be just fine.

Best regards and Good Health