WRONG EDD?

Yung gender ultrasound ko states na yung edd ko is august 30, pero bakit po recently lagi naninigas yung tummy ko and a little backpain. I am suppose to be 28 weeks now. Pero last prenatal check up ko sabi ng midwife my baby bump measures is for 8 mos. So kung 8 mos na ako, kelan po yung due date ko. I assume it would next month right? So ano nag kamali ng bilang. Oh by the way, wala po akong lmp since nabuntis agad ako months after my 1st baby.