I have always heard the heartbeat but in saying that the last appointment i had, my Dr (who is also an ob) had a medical student in with him and was explaining that hearing the placenta / cord flow was good in place of the heartbeat. I'm sure of your Ob was concerned she would have sent you for an immediate scan???
Big though to you cause like there isnt enough to worry about. Maybe if you are really worried call them first thing in the morning and get them to check again xx