Possible vanished twin

Hello ladies, I met my family doctor today at 11 weeks 4d. She reviewd my blood test reports and my 9 weeks dating ultrasound test and told me that I had two sacs. One was empty..sac sized at 7 weeks with no hb. The second sac sized at 9 weeks had hb of 178 and dated 2 days ahead of the due date. So, the doc checked the hb today and it is 158 at 11w4d. So, I am concerned now if the second baby would be okay going forward. I have a boy of 2.5 yr and no twins in my family or my husband's. I am sad that I lost one( doc doesn't know for sure if there was baby or just empty , yet) but worried about the living baby. Please share if you know or experienced something like that. My next ultrasound is in 2 weeks. Hoping everything to be okay.