6 week pregnant only seen yolk sac not baby yet

I went for my ultrasound I’m six weeks today they only see in the York Sac. Is this normal also I woke up last night cramping woke up this morning with brown spotting then turn into a dark brown spotting this is been on off all day and my back was killing me but on the ultrasound they showed a blood sack I guess behind my uterus has anyone ever had this before the ultrasound tech said this is probably why I’m spotting and the doctor will call me and talk to me about it later I’m just wondering what is this