First of all does she have a brother? Or any friends who have baby brothers etc.. She is probably 100% ok. heck you said this is your first girl?
Consider then. Boys can pee standing up. They do not have to wear dresses,. They have cooler toys, in cartoons or kid shows they are more likely to be the cool super hero.
Later tv shows that she sees will show men, largely fathers as incompetent etc... So just enjoy the fact that she wants to be a "Tom" boy and just have fun with her as she grows up.
Probably it is phase in a gender identity development but it can be a sign of a deeper problem.
I would suggest you consult an expert
Your little girl has seen another person, a male I would guess do this,could be another little friend, this is her copying it, check out who has been alone with her and be observant as they will do it again.and perhaps someone has shown them .
It's normal leave her alone try your best just to teach her to be a good person.