Talk to Gynecologist on Gynacologist

My lmp was 7th dec. I got my first ultrasound done on 28th jan and it showed 4 weeks and 5 days of pregnancy. Today again i got my ultrasound done and it showed pregnancy of 6 weeks +-4 days of pregnancy. Although i should be around 7 weeks 5 days today according to my first ultrasound. Is it normal? Am i thinking too much? Today yolk sac and fetal pole have seen but without heartbeat. Doctor have adviced me to wait for 2 weeks for rescan. Please help. I am way too worried