Need someone to answer this...help!

I know this is probably a dumb question... but if I am measuring at 4 weeks and 3 days based on an ultrasound, when was conception? I got a positive pregnancy test two weeks ago... I feel like this doesn’t add up. 251 hcg level one week ago. I’m so confused.

I also got a positive OPK dec 24th so is it possible that may have been incorrect and I ovulated later?