It happened to me too, on my LMP I would of been 6 weeks 2 days, but my dating scan came back at 5w1d or something silly like that. Then my ob appointment i was 8w5d LMP, and bub measures 8w3d. My ob goes off my LMP for my due date anyway.

I wouldn't be too concerned about it, your baby might have a growth spurt like mine did.

Congratulations!