Yolk sac but not seeing a fetal pole

Ok so I am suppose to be six weeks today. I have experienced some minor spotting and even passed a dime sized clot I went to my dr today where they did a ultrasound and blood work I go back for more blood work in a couple days and back for another ultrasound in a week all they saw today was the yolk sac no fetal pole no heart beat! I have had five miscarriages and I am scared I never experienced this and only went on to having spontaneous miscarriages my dr told me not to worry to much that I could just be to early to see anything... advice?