Gestational sac

Really hoping someone can offer some advice….

My husband and I had sex on January 15th. He has been on a work trip so I know that’s the only day. I’m not 100% positive but I think I ovulated on the 16th and got the first very faint positive on 1/24. On 1/25 HCG was 10 and on 1/27 it was 40. HCG has not been checked since but my pregnancy test in insanely dark to the point you can’t even see the control line any more. I am taking progesterone twice daily. On 2/9 via abdominal u/s at the OB they couldn’t see anything. On 2/10 I went for a private ultrasound because I was so upset and they saw a very small gestational sac that was 3.3mm which I think is pretty small for this point in gestation. I would be about 5 1/2 weeks. Has this ever happened to anyone else and what was the outcome. We have had a lot of early miscarriages and I’m so anxious. Going back to the OB next week but fearing the worse. I have no bleeding or cramping. Thank you so much for your help!