So worried even though doctor said I shouldn’t be 😭

Went in for my appointment today. I’ve had 3 now so far. Had one on 8/22 and sac measured 5 weeks 1 day... had another on 8/30 baby measured 6 weeks 1 day ( a day behind original estimate) and had a strong heartbeat. Had one this morning 9/4 and baby is measuring 6 weeks 4 days (so two days behind last ultrasound measurement). Anyone else going through this? This is my rainbow 😭 I just need everything to be okay. Doctor says not to worry but how can I not! I go back on 9/17. Should be 8 weeks by then which is so scary for me because that’s when my last pregnancy stopped developing 😭 say prayers for me mommas . I can’t lose another baby 😭😭😭