1 week difference! Help please!!

Katia
I've been spotting for the past 4 days! 2 days ago I went to the ER and they did an ultrasound. They saw a sac that measured less than 5 weeks. I'm very confused because I thought I was 5 weeks 4 days according to my lmp. My hcg levels were at 5950 at that time. I since then have gone with my dr who did another scan and said I was 4 weeks 3 days and they also did blood work to measure my hcg to see if they are rising. Has this happened to anyone before? I'm so scared to have another miscarriage ☹️ I go back for my results on Monday.