First Doctor Appointment

So this morning I had my first appointment, per my last period I should be 8w6d, but my periods are very irregular. When we did the transvaginal ultrasound she asked if I was sure my dates are right. I told her yes but my periods are irregular she showed me the ultrasound and told me that I was measuring 5w6d. We listened to the heartbeat and it was only 90. She said either I am early or that the pregnancy is not developing properly. She is having me come back in a week for another ultrasound, she also is having me get bloodwork done. Has this happened to anyone else and everything turned out fine?