I think it's a boy!
....or a girl.
Sorry I'm useless, and no idea about the nub thing. 
Some ppl on Mumsnet get excited about 'correcting' everyone and moan about them putting xxxxxxx/hun/hubby/lol etc, point out the difference between gender and sex, pick up on spelling/grammar mistakes and stuff on their posts, just have a look at some of the threads on AIBU or Chat and you'll see!
It doesn't bother me in the slightest, and you sound lovely!
We are honestly mostly a lovely bunch on Mumsnet, even those who 'correct' are lovely too, just look past it, brush it off and ignore op.
It's also difficult to convey the right 'tone' online too, someone might write something down very pleasant and to the point, but to someone else it might come across as sharp/bitchy etc.
You're right to be excited about your new baby, and it's only normal to wonder about the sex. Good luck 