Two weeks behind

So I had my first ultrasound today, and discovered it was twins. I’ve had a lot of bleeding/spotting since finding out, so this was quite the shock. Baby A measured 5w6d, FHB 127, baby B measured 6w, FHB 135. My main concern is that I got a positive pregnancy test Sept. 17, so it’s impossible for me to only be 6 weeks, and should be closer to 8. I have a small PG bleed, and the tech said it looks like they’re mono/mono, so I’m not sure if that’s affecting growth this early? Has anyone experienced they’re dates being this off and everything was ok?