Once In Awhile (inconsistently) My Heart Will Thump Once Hard

Once in awhile (inconsistently) my heart will thump once hard followed by an immediate shortness of breathe that lasts for one to two seconds at most as the air is sucked out of my chest (will stop me dead in my sentence while talking). This happens maybe once a month or sometimes twice in a week or sometimes not for 3 months. This started about two years ago. My heart pumps perfectly normal after this happens. Do you happen to know what might be wrong?