Is this normal?

Hi everyone I went into the ER last night for some cramping and shoulder pain. They said they thought they saw a small sac in the uterus which is great news and my HCG is doubling as well. My last period was Dec 22 which would make me 5 weeks and 2 days but I ovulated more around January 8th the earliest which would make me 4 weeks 6 days. My question is shouldn’t you be able to see a noticeable sac at this point. Is something wrong because the sac is not very noticeable at all. My HCG levels yesterday where 1,288.