PACs

I have been having an occasional irregular heartbeat in the past, it started in march.  I have occasional pac’s, they seem to come on during times of high stress or extreme physical exertion.  I am a firefighter/paramedic and I notice them mostly when I work a house fire but at times they come during my training.  I have no pain with them.I have seen a cardiologist and had an echo and 12 lead done.  The echo cardiogram came back normal according the the cardiologist.  my 12 lead says that I had an anterior septal infarct (old).  My cardiologist says that I have nothing to worry about and that pac’s happen to some people for no reason.  I guess i just feel uneasy about these since they kind of started out of the blue and they come so infrequently.Do I have anything to worry about with these, could they be a sign of something else?