Confused Can someone explain to me something about pregnancy

Ok can someone tell me something How does pregnancy work ok when i went in they said because of my last period april 19 ill be due on jan 25 but when dr did sonogram shes like your way early her guess was about 2 weeks and i know pregnancy is complicated the way its figured out and if someone could explain they didnt see anything on sonogram to do measurements. They have me coming back for another sonogram on friday. so what am i am i 2 weeks or going on 5 weeks they wrote in my chart jan 25 im so confused please help