U/s tech said I was 17wks&4 days based on lmp, but then said I was 18 weeks once she started the ultrasound. Why did that change? What did she go by?

No change. Prenatal ultrasound is not that accurate at 18 weeks(+/-7-10 days). Your menstrual dates should be used if the difference is within that range of confidence.
See my other answer. It is hard to tell. There are numerous measurements involved. (head circumference, abdominal circumference, femur length, humeral length, biparietal diameter, etc etc....).
Measurement. When she measured your baby the age of the baby most likely changed since the ultrasound measurement was larger than your date based on lmp. This is normal.
Educated guess. It is not unusual to get two different answers when using two different methods to determine gestation. Neither is exact, they are guesses based on history and what your technologist is seeing. Please discuss this with your ob/gyn as they have more information with which to make the determination. Due dates often change, and in the end, it's mostly the baby that determines when it is time.