GESTATIONAL SAC DISAPPEARED ?!

I’m supposed to be 5-6 weeks along. They saw the G.Sac Monday and Wednesday, we’re concerned because only the sac and no yolk or baby.

My hcg on Wednesday was in the 13,000’s - Friday was 17,000’s.

Then today I had an ultra sound and they saw no gestational sac anymore??

Wednesday night I was in a lot of pain and was crying but didn’t go to the er. Woke up Thursday and I’ve been fine.

I do have random sharp pains in my vagina stomach area.

Why would the sac disappear?

If I was about to miscarry wouldn’t they see the sac still or ?