Big fat positive to big fat ???

Attached is my logged periods, possible conception and I believe ovulation was around 2/6. I got my big fat positive for our Rainbow baby on 2/15! 🌈 I have irregular periods ranging from 20-30 days in between.

Today I am supposedly 8 weeks and I went in for my first ultrasound. I was measuring smaller than expected, just like my last 2 pregnancies (1 resulting in a miscarriage). The Dr estimated me to be only 6 weeks based on my gestational sac, however that was all that was showing on the ultrasound. Just a 6 week sized, empty gestational sac. No yolk sac, no fetal pole, nothing. I am so confused! The doctor says not to give up hope yet and another ultrasound in 2 weeks will confirm viability of the pregnancy.

Can my weeks be so off, when I had a BFP on 2/15 and now nothing? Has this happened to anyone else and had a viable pregnancy? I’m so anxious to have another miscarriage. 😣