confused?? measurements

So i had my anatomy scan friday when i shouldve been 18 weeks 1 day. My due date is March 2nd and it has stayed that way since 7 weeks at my first checkup. Same at my 13 week ultrasound. When she was doing my anatomy scan she said she was measuring me at 17 weeks? which i didnt expect at all but she also said the baby wouldnt roll over onto her side so she could get pictures of her spine. she said they do these ultrasounds at 18-20 weeks and since the baby wouldnt roll over i would most likely have to come back in a couple weeks to get the pictures they needed. i thought i was 18 weeks already thats why i scheduled the ultrasound cuz the doctor told me 2 weeks before to schedule it in 2-4 weeks when i would be between 18-20. & another thing, i asked her would my due date change now since the baby was measuring 17 weeks and she said no that it had nothing to do with the due date? so now i dont really know of i should change my glow or what i would even change it to because all she said was "im measuring u about 17 weeks" and wasnt very specific about anything. and i also never heard of measuring a week back and the due date not changing? can anyone explain this to me or help me understand better or was this lady possibly wrong? im just confused