How far along am i

Trying to figure out how far along i am! My app and the dr thought i was 6 weeks definitely not had to go in today for an ultrasound she said based off my beta numbers we probably wouldn’t see anything on the ultrasound; she was right. She said she wasn’t concerned. I tested positive on January 7th we had sex December 16th,18th 22nd, 25th, 27th, 29th is there anyway of figuring out how far along i am? I’m worried that I should be further along but possibly no growing baby?!