6 weeks ahead ?!

ok so at the beginning my dr thought i was due in september based off my last period. i took at test on valentines day and it was negative, i took another 2 weeks later and it was positive. i know my body pretty well so i assumed i was 6/7 wks at the time of my positive test. my dr said i was 14 based off my period so she did a quick lil ultrasound and baby looked like a lil peanut and measured 7 wks 3 days not 14. so she said your due nov. 5th. i have been going off that since april. my appoinment yesterday took a HUGE turn when she measured me and said im measuring 30 weeks instead of 24. so now shes still saying im due in september. i just cant wrap my head around being due in nov to now sept. im hoping my uterus is just really big since this is my 4th baby cuz i am not ready to have my baby in 7 weeks.