It's SO frustrating isn't it! I'm in the same boat. I don't know about your nub. You were only 12 weeks and that is very early to tell. My midwife sends her patients to an ultrasound tech that says she can definitely tell by 13 weeks. I am on baby #4. I had 2 girls then a boy. We desperately wanted another boy to complete our family. We did everything to make a boy, even iced my husbands junk to kill the girl sperm.
I could not stand waiting to find out the sex so my midwife said I could get the US at 16 weeks. I went to the place covered under my insurance instead of the place where the woman can tell the sex by 13 weeks. We had a good view from the bottom between the legs and I saw nothing! The tech though it was a girl. When my family came into the room to see the baby I got a quick shot from the front and saw the nub! The tech saw it also and said she thinks it's a swollen labia. She said 16 weeks is a little early to tell. I was so upset! If they can count the chambers in my babies heart and are suppose to check the development of the sex organs then why the heck can't they tell me for sure if it's a boy or girl? I asked if she could see ovaries, and she said it doesn't work like that.
SIGH! I'm waiting until I hit 20 weeks (2 weeks to go) then I'm going to pay out of pocket what ever I have to, and go see this other ultrasound tech for the sole purpose of knowing the sex. I WANT to see the 3 lines of the labia and have visual conformation it's a girl, or see a penis and scrotum and confirm a boy. Not the dreaded "think/maybe".
This site gave me the best reference: