Help!!

I finally got my BFP!! My last LMP was August 22nd, but I only bled for 2 days very light to the point I didn’t even fill a tampon. This would date me at about 7/ almost 8 weeks. But I was unsure, so I went in for a dating scan and the doctor told me by looking at the ultra sound they concluded I’m 5 weeks and 6 days. But I’m confused on how that makes sense. Did they dare it by my LMP or by the fetus, it’s just not adding up and I’m a little confused. He said he saw a healthy heartbeat also. Are these good signs? Am I actually 7 weeks? If not how am I 5?