Sometimes I feel my heart beat or jump hard, one beat and the continue normal. Why that happen?

Premature beats. You seem to have occasional pvcs (premature beats followed by a pause and a hard beat). They are generally of not much concern if happening once in a while. However please talk to your doctor about it.
PVC. If t is an infrequent, brief event with just one abnormal beat it is probably a pvc. Premature ventricular contraction means the ventricle part of the heart delivers a beat before the normal pacemaker. It happens very commonly. Low potassium and other electrolyte problems would make this more common. The cause could be significant but the PVC itself doesn't seem to cause problems.