How far along am I?!

Charise
Ok so at my Feb. 21st appointment I was 10 weeks. Awesome. Well at my March 20th appointment the nurse said I was only 13 weeks I'm so confused because I thought I was 14 weeks. 
It's also important because I wanna do the 3D ultrasound but you have to be 16 weeks which I was hoping was next week. 
Can anyone help me feel less crazy?