Twins and measuring earlier than estimated

So, I just had my first ultrasound, and according to my lmp I should be between 8-9weeks. However, the tech dated me at 6 weeks, and we discovered that it’s twins. I am still breastfeeding so she thinks I ovulated late because of that. Has anyone else had this happen? Where they’re measuring two weeks behind where they thought they were and everything was ok? Is this normal for twins? I’m still in shock and kinda freaking out.

I should also add that I got a positive home pregnancy test on Sept. 17, so I can’t be less than 7weeks. Both babies had a heartbeat 127 and 135, so I’m hoping that’s a good sign.