Have had exactly the same thing, heart seems to pound every now and then. Did a bit of a read, says it's normal in pregnancy for your resting heartrate to increase. My resting heartrate pre-pregnancy was 45bpm, me and the man timed it at 70bpm one night but you've got a lot more blood pumping around so it's not that surprising! I'm doing good according to all my docs and midwives though, so I think it's unlikely that you need to stress either.