Sometimes I'll be going to sleep, or watching something, and my heart will give a single really hard thump that will make me feel like coughing. This has only started happening in the past year (I am 19). I'm moderately fit, I lift weights and run regularly. Other accounts of heart palpitations report that it happens several times over a certain period, E.G. happening every 3rd beat over the course of a half hour, but for me it is not like this at all.
To simplify, maybe once in a couple of months i'll have a very random very hard heartbeat that will make me cough, and I have no idea why.