I've often felt my "heart beating out of my chest since surgery 12 days ago. Sometimes in the throat, sometimes the gut. I THINK some bass-drum heartbeats are the main reason why I'm still having trouble sleeping more than 3 hrs. Several times I've awakened with a start, with the distinct recollection of having been "smacked" by a big strong beat. Mine are in the throat more often than the gut, but I've felt both, and it seems similar to me.
When I was still in hospital and monitored, they were detecting fairly frequent PVCs = premature heartbeats. They seemed to coincide, at least partially, with the big strong beats. AFAIK, I didn't get those pre-op, or at least pre-symptoms.
I have been known to focus on my heartbeat for decades, partly because I used to enjoy using "biofeedback" to lower my resting heartrate, just for fun. So I don't think my changed mental focus is a big part of the change.