No heartbeat again .. possible twins

Hi .

So went back for my second ultrasound .. still no heartbeat . Dr said it may look like twins in the same sac / really close together . Looks like maybe 6 weeks measuring 4.2 Crl . Should I be worried :/ ? She said it looked abnormal and never saw anything like it before ... she wants me to go back on Friday to find the heartbeat she said it should be found at this point. ...the left photo is 2 weeks ago .. right was today . She said it barely grew only 5 days :/