6 weeks and 1 day pregnant but did not see a heartbeat?

i went for my first ultrasound on monday. im suppose to be 6 wks and 1 day. they did a l ultrasound and we saw a sac and everything seems fine. i thought i saw a flickering but it was going kinda slow. the doctor checked my pulse and said she couldnt tell if it was an echo of my heartbeat or the baby's. she took some mmeasurementsand said the baby is the correct size. i had 2 blood draws 2 days before and my hcg levels were rising. im suppose to come back on tuesday but im am freaked out because i had a sonogram before with no heartbeat at 6 wks and i had to get a d&c. im really worried. is this normal. pleased help. i know you cant HEAR it at this time but im suppose to be able to SEE it right?