Measuring 3 weeks behind..

So based on my LMP I should be 10 weeks tomorrow. I had a bleed so went to EPAS for a check. They’re saying I’m only 6+6? Making me 3 weeks behind where I’m supposed to be? I don’t see how this can physically be possible? As counting back 7 weeks is the week I tested positive 🤣? They’ve adjusted my 12 weeks scan by 3 weeks and I just can’t wrap my head around it? How can I be 7 weeks if I tested positive 7 weeks ago and my period was 10 weeks ago…