Eptopic pregnacy

Going by the last day of my cycle which would be 7/26/15 im suppose to be 6weeks and 2days... I got an ultrasound today that showed im only 4weeks. Im nervous because the dr didnt see the yolk sak. She either im really 4weeks or my pregnacy stop progressing at 4weeks or its eptopic im so upset im keeping the faith that i still am progressing with a healthy pregnacy. Has anyone wxperience this?