Because you're so thin? That doesn't really help predict the sex of the baby. I had my AS with DS at 23 weeks. I was also quite thin if that matters to the story. I was told with 100% certainty that it was a girl. It was the best "hamburger" shot she'd ever seen. This is a veteran tech, so not a recent grad. I didn't believe her because I was sure it was a boy. I had already told people it was a boy, that's how sure I was. Mistakes happen, but after 20 weeks, it is almost unheard of. Lucky me.
At 30 weeks, guess what? Another tech got the best "turtle" shot she'd ever seen.
Chances are, it is a girl, but mistakes happen. When I was pregnant with dd, I made them "prove" it. I had several ultrasounds, and I made them tell me each time, lol. After the fifth tech told me it was a girl,I was confident.