Gestational age ng last mens date at ultrasound di matched

August 24 1st day menstruation ko. Sept 20 nag pt ako, negative. Sept 28 pt ulit nag positive. Oct 11, I went to OB for transV ultrasound. Just saw the gestational sac without yolk, so I was told to come back after 2 weeks, Oct 26 may amneotic sac na pero di pa madetect ang heartbeat. So babalik ulit after a week. Based sa date ng last mens ko, aug 24, 9 months na sana ako. Pero sabi ni OB, based sa ultrasound 4-5 weeks palang. May ganyan po ba talaga na di matched? Since nagbebase EDD usually sa last mens. Friend ko 3 months and 1 day last mens nya and matched sa ultrasound nya na 3 months old na. Anyone na same situation ko? Do I have something to worry about?