If due date was june 3 based on lmp of aug 27, then it changed to may 31 after u/s, could this be bcuz i ovulated sooner? conception date different?

Yes to both. : Is very common to have the due date changed based on the findings at ultrasound. It is often around a month off because it is frequently such that one does not ovulate when they thought they did, but actually ovulated the previous month or in fact a later month. So your findings are common and should be believed because the early ultrasound is extremely accurate.