concerned

Ok... So I got my positive pregnancy test on August 28th... My last period was July 17... I've had 2 ultrasounds 1 week apart and they don't see anything but a sack. The Dr says I'm deff pregnant but it's too early to see anything... I'm just very confused... My last pregnancy, which unfortunately ended in miscarriage, they were able to see and hear the heartbeat at 5 weeks... Why is this so different? I'm trying so hard not to stress out but it's getting harder everyday. I'm just pretending that I'm not pregnant (other than eating healthier and drinking more water) so I don't get my hopes up. I haven't even said anything to my husband.. I don't want to break his heart again if something goes wrong.