ER ultrasound question?

I went to the ER last night for some bad cramping I've had for about a week now but yesterday I had discharge? Brown blood? I wasn't sure. When I got there they confirmed my pregnancy & checked for infection; I am 6 weeks & 4 days pregnant & no infection. When the doctor did an abdominal ultrasound all she saw was what she THINKS was a yolk sac but maybe not. If I am 6 weeks 4 days, even on an abdominal ultrasound shouldn't more have been seen?! I am concerned after reading about blighted ovuma & missed miscarriages. Could someone please give me their opinion??