13 days behind

I went for my 8 week scan on Monday. I was exactly 8 weeks. I just got the results back and my doctor said I was measuring at 6 weeks 1 day. He was very positive about the pregnancy as the babies heart rate was 132 bpm and they pushed my due date back 13 days. I'm just pretty worried of course. A week I would get. 2 weeks are a lot. That would also mean I got a positive pregnancy test (very very very faint) at 2 weeks and 4 days. Which I don't think is possible. He said do another scan and as long as I grow and meet up with the new due date then who cares really. My HCG was 8019 and went up to 13348 after 5 days. Does that seem slow? My doctor seemed positive but this is my 5th pregnancy (2 boys and 2 mc) my last mc was a few months ago so as you can tell I'm pretty worried.