Scared and confused

I had my lmp July 11. Got a bfp August 9th. I went in for an ultrasound August 27th and was told they only saw the gestational sac. I have had HCG levels checked they kept rising last one 9/1 over 68,000. I went in for an ultrasound today baby only measuring 6 weeks 1 day. I should be at least 9 according to my lmp. I’m my HCG was checked today and down to 48,000. I return in two weeks for another ultrasound. Has this happened to anyone else and had a positive outcome? I have so many emotions going on.