Ultrasound wrong?

Anyone else have experience with getting wrong gender results during ultrasound? I already have a four year old son and going into my first ultrasound this pregnancy I didn't have a preference either way, but they told me it was "definitely" a girl. So I spent the last four weeks getting excited about a girl- we've already painted the nursery and everything! Today I had a follow-up ultrasound and it is "definitely" a boy. 😳😳😳😳😳😆 I am in serious shock. How do you just grow a winky in 4 weeks!? The top picture is the one from today on top and the one from 4 weeks ago on bottom.