Totally off on dates?

Have any of you ladies thought you were spot on with tracking etc, then come to find out your due date is way later than U thougt? According to my lmp and cycle length, I should be 8w3d and due oct 21, but when j went to the doc on tues I was measuring 6w1d which makes no sense to me at all! Even my doc is confused because my hcg #'s are that of someone that is 8weeks ... Agh.  I hope everything's ok!