Worrying Ultrasound

I had an ultrasound yesterday and my doctor is a bit worried about the results. I have a history of losses, I had 3 miscarriages in a row before carrying my son three years ago. Based on my LMP, I should be around 7 weeks tomorrow, but my ultrasound indicated that I am only about 5 weeks along though it did show faint fetal heartbeats. I’m worried that this likely means the fetus isn’t growing at a proper rate. I tested positive 3 weeks ago, and I feel like it’s impossible to have tested positive if I was 2 weeks along at that point, so my g he as is that things aren’t developing properly. I know it’s unlikely to have a miscarriage after seeing the heart rate, but that has happened to me several times so it isn’t super reassuring to have seen that.