First ultrasound


So According to my LMC on March 20 I'm suppose to have 7weeks and 5 days. I went to to my first ultrasound today. At first they tried to do an abdominal ultrasound and fail so the nurse told me to go to the bathroom and empty my bladder. Then she did a vaginal ultrasound and spent around 20 minutes looking at every. I new something Was off. Then she told me ( you are barely pregnant). I asked if there was no baby. She said oh yeah there's a baby and a yolk sac is just that the baby is measuring 5 weeks and 4 days. She also told me that there's a heartbeat but is slow and it was completely normal cause it was too early. She said that everything looked normal according to gestational age. But still how come I'm 2 weeks off? And I got my BFP on April 16 when I missed my period. So that means that I was supposed to be 4 weeks on April 16th. I'm sooo confused.