I'm so confused

This is so silly but maybe someone can shed light. I'm 6 weeks 6 days according to lmp. I went in for ultrasound today and baby is measuring 6 weeks and 3 days. So that means I conceived day 3 of my period you would think except I don't have sex on my period. That's my week off. So is baby just measuring big or did I really truly somehow conceive on my period? Thanks I know it's silly and all that matters is that I am pregnant and baby had a heart beat and is perfect. Just left me confused.