spotting at 5 weeks

I was spotting brown discharge on Monday dr. Sent me for an ultrasound found gestational and yolk sac and no fetal pole measuring 5 weeks 6 days which according to my lmp I was exactly 5 weeks. They are checking my hcg which yesterday was 18082. I get a recheck tomorrow and an ultrasound on Tuesday again. Should they have seen the fetal that Early? I am scared and a bit confused....