Why does my heart beat an extra beat after exercising? please try to answer.?

These "extra beats" are ectopic beats, aka Premature contractions, which happen when another part in the heart than it's natural heartbeat initiator initiates a heartbeat too early, causing a "flip-flop" feeling and sometimes a short pause. They are normal and increase during adrenaline rushes. They sometimes (but not necessarily) cause dizziness and other minor neurological symptoms and in some cases with a sick heart (like Myocardial Infarction) launch a life threatening arrhythmia.