Is this possible?

I got a very distinct positive pregnancy test five days before my missed period. I did opk's way before I got a positive opk result.  My question is, Is it possible I am farther than I thought? (my friend said the same thing happened to her, when she went to the doctor she was two weeks farther along than she though. )I tested positive on the opk so wouldn't that be the only day I could have conceived?