I was told that i am due on june 13th?

So that means i am 31 weeks right now but when i went in to measure me they got 35 and a half weeks with the tape measure. Ummm I have to go back in a week to get a utralsound done to see how big the baby is!! I know when they measure it is suppost to tell u how many weeks u are so are they off on my due date. P.S. I didnt have a period since april because i dont have a period every month this was a surpise that i even got pregnant because of no period since april. Can anyone asure me of if i am 35 and a half or could the measure thing not be 99% right??