I am a 21 year old female student and I am in pretty good shape, but when I am running or doing exercises like jumping jacks my heart feels like it skips a beat and I feel a thud in my chest. I don't get dizzy or light headed, but i do feel an urge to cough to clear the skipping. After a moment it resumes its normal beating, but it scares me so much I stop running. When I am doing other workouts like walking, biking, or anything that does not require me to bounce up and down I never get these skipped heart beats. What could i be?