Gender change

I had originally gone to a free clinic where I got my first ultrasound at the beginning of my second trimester, they were able to see that it was a BOY early on (baby wasn’t shy at all) and showed me the genitals and how it made him a boy. But now I went to a normal clinic and when they checked the ultrasound, baby was at an awkward position so the nurse kept poking/shaking my stomach to get a good view but said she couldn’t, so she gave up and said it was a GIRL. I’m happy with either gender but thought it was weird, has this happened to anyone?