During ejaculation sympathetic system is active. It is a normal process which happens to everybody. It is the same system which makes you run when you see a snake or when a dog is chasing you. So your heart beat increases and your breathing also becomes faster. Palpitations are a normal reaction during the climax of the sexual cycle (during ejaculation).
When you worry about the palpitations then the anxiety
increases. (That is because palpitations are a sign of anxiety and you are anxious about the anxiety itself). You end-up in an endless loop of increasing anxiety and palpitations.
So next time you get palpitations, just tell yourself, "This is a normal reaction" and observe the heart beat closely - you will find that it immediately slows down.