If your doctor siad not to worry, I wouldn't worry too much. 11 weeks can be early to find a heartbeat with a doppler, especially if you have an anterior placenta (placenta in front). Maybe you could have your next appointment moved to the place that has u/s and if he can't find it again, have a u/s then.
DD#1 and this baby both had anterior placentas and they couldn't get their hb on doppler, I had to have u/s to check.
SAHM of amazing DS Quentin
(03/07),rainbow DD Audreanne
(11/12) Angel Gabriel
(04/08), plus four more