you aren't being silly, go get you another u/s! After about 18wks, ultrasounds are usually 95% correct. But there are circumstances where things can get in the way and depending on the experience of the tech and machinery, cords can be mistaken and so can fingers as a penis.
We wanted only two kids too. I have two boys. who are 19mths apart. Ages 3 and 1 right now. Accidentally got pregnant this time around! but it turned out to be a girl for some reason...
Having two little boys has been such a wonderful journey. I'm so excited about the future with them growing up close together. I wanted boys after watching Home Improvement growing up and the Sandlot!! ;)
Congrats on your pregnancy! And please update how the u/s goes!! see if they'll give you a gender shot pic