Measuring behind

Long story short , I went in last Friday and found out I was supposed to be having twins , but one of them didn’t make it . They couldn’t get a reading on a heart beat for the other baby so they rescheduled me & told me to come back yesterday for another ultrasound. They told me my body had passed one baby and the other baby did grow from the last ultrasound but they still couldn’t find the heartbeat . I was 8w5d and the doctor told me I was measuring 6w1d and that usually twins are smaller . They did check my hcg levels and they have went up . Has this happened to anyone before ?? I’m terrified that something will be wrong with the baby because I’m measuring a few weeks behind ...