Drastic due date change

My first appointment, the one where you just confirm the pregnancy and get set up for the rest of the appointments they used the wheel to determine that I was 13 weeks then. My first actual visit with the doctor I went in thinking I was 21 weeks but measured 8-12 weeks. I've been feeling what I thought was movement for weeks now but if I'm only 8 weeks then how have I been feeling these twitches and flutters for the past 4 or 5 weeks?