Could my dates be wrong?

ok so i got my bfp over two weeks ago and im in my 7th week. with my other pregnancies i was extremely sick from the 3rd week on, this time though i barely have any sickness just occasional nausea, sore and very tired but nothing near what i had the other times, i go for my ultra sound in 5 days but do you think maybe its becoz im not as far along as i think i am?