Pregnancy Help please

Hello.Im approximately 6 weeks 1 day pregnant and I went for my first ultra sound today.My HCG levels went from 762 to 3666 in 4 days but today I went for the ultra sound and nothing could be detected on that so we did a probe and they were able to see the gestational and yolk sack but no fetal pole or heart beat...The doctor told me it was normal for there not to be a heartbeat because it too early but I looked it up an they say at 6 weeks you CAN detect a heart beat...I have to do a follow up in 3 days to see if my HCG rises but I'm soo nervous.Anyone else experience this?