I had my appendix removed at 11w3d and my son is three now. Take it super easy and try not to stress that's really all you can do. Good luck!
Yes i had the ultrasound 2 days after surgery. The u/s tech had said that it was too early to tell if it was a boy or girl but then my baby flipped around and pretty much put the groin area right on the screen. He asked me if i wsnted to know what it was. He said it might not be. 100 percent accurate but he has been doing this for over 20 years so he said he feels 90 percent sure it will be a boy
if your only 12 wks it sounds really early for her to be certain that its a boy. My tech said she could not tell until after 16wks and that they dont tell you until after 18 to be sure.
As for the heartbeat I know that sometimes they have to chase around the baby to find it since its not always in the same spot but you had your U/s after your surgery right? so try not to stress out they saw the heartbeat.