As other people have suggested, it seems like you are experiencing some premature ventricular contraction
(PVC), also known as ventricular premature beat
(VPB) or commonly as extrasystole. They are a form of irregular heartbeat with a "skipped beat" followed by a stronger beat. They are often reported by people who experience them for the first time like: "the heart stops".
They are very common, and can occur in both individuals with and without heart disease
In people without heart conditions, PVCs can be triggered by use of cocaine, amphetamines, alcohol, tobacco, and caffeine, as well as medicines like digoxin, sympathomimetics, tricyclic antidepressants, and aminophylline
. They can also be due to hypomagnesemia
, hypokalemia, and hypercalcemia
PVCs can also be triggered simply by exercise or anxiety/lack of sleep.
In your case, due to the fact that you are experiencing only very few PVCs a day, my best guess would be that they are a sporadic event, maybe stress-related. If you are able to do so, monitor your pulse, and see if they actually are more frequent than you feel. I can't give you a medical advice
without seeing you, but my guess would be that they will probably stop soon. If they don't or become more frequent, or if you develop other symptoms or simply if you're worried, see a doctor, who will be able to reassure you.