Confused about how far along I am

Nicole • Been married to the love of my life for almost 6 years. We have been together for 13 year. Our baby girl is now 11 months old
I went to my first appointment and they said I am 4 weeks. I didn't ovulate until the 21st. How it is that I am already 4 weeks? She said it's bc they go by the last day of my last period. But why?